Ugh, another tire question

I have a 77 GS-400, front wheel is a 1.60x18, rear is 1.85x18. The tires are currently Avon's 90/90/18 f, and 110/90/18 r.

I like the fatter tire look and I'm looking at the IRC GS-11. Can I fit a 3.5x18 on the front, and 4.00x18 tire on the rear?

The chart on IRC's website call for a 3.00 front and 3.50 rear on the GS-400, but I'm assuming this is concervative considering the sizes I have on my wheels now.

Has anyone done this?

Nope, way too big. Check this chart out:

http://www.dropbears.com/u/utilities/tyrerim.htm

You can easily fit wheels from other GS's though. Only problem is the 18" wheels you have now are the only widths (that I know of) that were made.

so why is it a 90/90/18 f, and 110/90/18 r fit no problem?

Just for referance. http://www.irc-tire.com/mce/

Ultimately it's up to the tire manufacturer to test their tires for fitment to wheels. Check Avon's site- I bet they don't recommend your current tires for your wheels.

Would it be possible to lace rims from another bike onto my hubs that would fit a 4.00x18 front and rear?

The Suzuki GT500 has a 2.15x18 rear wheel, now for the front?

Here are a few of my inspirations for the wheel/tire look im going for, just to give you all an idea. Notice the tire/wheel size is the same front and rear.

If your bike came with spoked wheels then changing to different widths is limited by your wallet and how much space you actually have for tires.
Nab yourself some take-off 18x2.15" or maaaaybe 18x2.5" if you suck you're gut in when building the combo...but you really want to check your clearances. Just because it clears with an 1/8th when it's just sitting there means nothing when it's actually rolling under 'power' and the tires shift, and the suspension flexes which is will on a GS400 or any other bike of that general style. When that flex does occur the tires can rub, sometimes pretty drastically.......would suck to have a wheel try and lock up or even slow when you're healed over in a corner......