A good set of Shinko 230/SR741, Pirelli Sport Demon, or Avon AM26 RoadRider tires will make the GS handle awesome, considering it already has the best handling frame of any Japanese bike built prior to 1979-1980. Wider rims on the spoked wheels will help more with powering through corners at serious lean angles, as a 110 or even 120 is not quite sufficient to keep from doing some drifting burnouts while on the throttle harder mid-turn (these engines make GOBS of horsepower from 5,000-11,000 rpm especially with K&N or Uni filters, a 4-1, and rejetted carbs - look to theGSResources.com or me for GS750 rejetting advice).
You can get really nice RK/Excel H-section polished aluminum rims in one size up from stock in 18" front and rear (2.50 rear/2.15 front) drilled for the proper spoke angles for our hubs (GS400 spec), from Japan.Webike.net for $530 with a set of stainless GS400 spec spokes that I "think" will still work with those H-section rims, $530 shipped from Japan! That's a good $250 cheaper than even Woody's Wheel Works would want for Excel Takasago's in a larger rear rim and stainless spokes. Buchanan's would be nearly a grand for that.
http://japan.webike.net/products/21724568.html
http://japan.webike.net/products/1522879.html (stock GS400 spoke length, I am guessing this would work with the 18" GS400 H-section rims above)
To get rear positioned rearsets, you pretty much have to cut and weld on the frame, or else make a big aluminum plate style mounting bracket to move them more rearwards.
You can get all sorts of great shocks with the clevis mount on the bottom for these bikes. the $200 progressives and Hagon's are the cheap go-to shock for a decent ride. The Hagon 2810 is adjustable dampening for $300. The Ikon 7610 is a rebuildable adjustable version similar to the Hagon 2810. My recent favorites are YSSUSA.com's E-302 (standard alloy rebuildable shock ($349), C-302 Piggyback version ($449), fully adjustable dampening Z362 ($499), and the G-302TRL piggyback with adjustable rebound dampening ($599), or the G-302TRCL piggyback with fully adjustable compression and rebound dampening ($699). Good stuff that a few builders on GS Resources have picked for their racer projects.
Also, Fox has just started making some of their new twin shocks tailored for our vintage bikes, and the adjustable non-piggyback version is around $600 and is top top notch quality up there with Works, Ohlins, etc but slightly better price than the high high end stuff. See the vendors forum section on this website from Cognito Moto I think it is, they worked with Fox to make a shock more tailored for our bikes.
YSS Piggyback's:
In the front, MikesXS.com cartridge emulators and Sonic Springs work awesome to modify the old forks, and Tarozzi makes a great fork brace for these for $99.
Also 93/94 GSXR1100 or 1000 (?) forks are a great swap in (correct height), and even better are 98-00 Honda SuperHawk (Firestorm outside the USA) VTR1000F forks, which run the same size rotors that guys upgrade the GS's to (296mm CBR900RR etc rotors) and therefore would be more easily adaptable to the stock GS hubs since the CBR rotors are nearly a bolt on mod.
On the GS forums, see salty_monk's thread "twinpot upgrade on the 78 skunk" for the Kawasaki Ninja and Honda CBR/etc dual front disc upgrade mod. The VTR forks are better for retaining the original wheels like I said (minor custom fitting may be needed with axle/spacers), and have adjustable dampening. I am running the stock 35mm forks still and my bike is handling quite well with these other mods and the brace. a 37mm GS1000 fork may have been a wiser choice for me when I was upgrading to dual disc, but I have zero qualms with the 650/750 forks that I have in 35mm, and I ride this thing very hard deep into the Appalachian twisties in the hills of southeast Ohio. Below is the brake mod that many GS Resources Forums members run thanks to Dan aka salty_monk researching what rotors and calipers work best. Mine is customized, normally the lower bolt hole requires a bracket that salty/dan sells, as well as most of the Honda donor rotors requiring a spacer behind them. The "599" or "Hornet" depending on the export country has rotors that need no spacer at all, because they have a 22.5mm offset already. Most of the Honda rotors that are useable have a 17mm offset that requires about 5mm of spacing behind them. Haven';t ever seen a used Hornet set yet, but many of the 17mm offset rotors sell for very cheap used.
You can also get excellent braking sticking with the single disc fork legs with a higher caliper mount by running one of those above pictured Ninja/GS500 left side calipers (the Ninja 250 and late model GS500 may use only a single right, can't remember) and salty's bracket or a custom modded bracket like mine in the photo (bottom bolt is now a direct bolt in also), and run a CBR900RR Fireblade or CBR1100XX Blackbird 310mm rotor and have a much lighter and much more massive floating rotor with a sliding two piston caliper and longer pads than stock. This is really plenty of sufficient stopping power, but if you race it (running full speed into corners nonstop and braking hard) or run in the mountains all the time where there are tons of twisties and switchbacks on long downhills, a 310mm single rotor may get a bit hot and warp. This is only under extreme use, mind you. It's plenty of braking power for most.
Seems like you have made some progress on tackling this build already, but most of your photos are not displaying for me! What's up with that? mobile and desktop viewing. Beautifully clean well preserved example of a GS750, I must say as well as all the others.
If you want to put that exhaust in a museum or sell it to an envious owner looking for a nice stock set, check out Marving's flat collector upswept road race pipe 4-1 for our bikes. They are Italian, but they can ship to the US and I think they have one USA authorized dealer. It's AWESOME, better than a MAC or Vance and Hines or a used or NOS Jardine or other pipes that used to be available for our bikes. It's on my dream list of parts I am having trouble justifying upgrading (I have both a MAC and V&H currently).
If that is a 1977, the rear brake is overly sensitive and could stand downsizing to a smaller rotor from a 78-early 80's GS750/1000/1100, and matching caliper hanger bracket. way too much rear rotor for a Japanese inline four chassis. I went with a 90-ish Katana 1100 front floating rotor for mine, as did a fella on here with an awesome 673cc GS550 cafe build
http://www.thegsresources.com/_forum/showthread.php?193507-GS-550E-1980-cafe-build/page7.
Best of luck, keep this project looking great!