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I think the Benly Dream CA72/77 has a different crankshaft, ignition and camshaft than the CB72/77 Hawk / Superhawk. One is a 180 crank and the other is a 360 crank. So, you would have to change the crank / cam / ignition, so maybe a better bet is to send a Benly cam to a cam grinder and get it reground to whatever specs you want.
Back in the '60's I'd heard of - but never saw - CA77's with complete Superhawk motors installed, but don't know the issues about carb clearance, etc.
As far as CB350 pistons, check the compression height of the piston against what you already have. Something else you may consider is looking around for a Superhawk 350cc big bore kit - it should work with your cylinder. The pistons in the kit have the correct compression height, dome shape, etc., for your street bike. NOS kits are still around if you look hard enough. The CB77 racing kit pistons offered by some vintage race shops may have too much compression for a street bike, but you could check with them.
As far as the 26 mm carb - one Superhawk carb is what you're talking about, right - sure why not but it will probably be easier to get a Mikuni around the same size and Mikuni jets, needles and other carb tuning stuff is far easier to find IMO.
As far as doing the ton, if you're looking at it seriously, start on a major weight reduction program - the Benlys are pretty heavy. You could probably lose 40 pounds pretty easily, without any major cutting (think front fender, seat, electric starter and install the smallest battery that will work with your charging system (or remove it, too, for more weight savings). Add clubman bars to get down out of the wind as well.
Nice ride, BTW.
Just my 2 cents worth
You want to do the ton on a Dream? Seriously? That could be a nightmare....The Dream frame is stiff enough but make sure you have brakes and decent suspension at both ends. You could fit a CB77 or 450 front brake I guess.
CB72/77 cams are better than Benly but not enough IMHO. As PacoMotor said, get Megacycle to grind you some cams. You will need to lighten the rockers (preferable but not 100% necessary) and fit R&D valve springs. You can keep the 360 degree crank but they make less power than a 180 crank as Honda discovered by in the day. They called them Type 1 and type 2. But for your purpose, a good stock crank will be fine. And fit a new cam idler wheel. Old ones go hard and explode when the revs go up.
CB350 pistons can be machined to match a Benly/Superhawk combustion chamber and the valve pockets have to deepened. I used them for years in CB77 race bikes. A few years ago there were a couple of guys selling pistons to take those motors out to 64mm bore. I don't know if any are still available though. They typically have thinner rings and that's an advantage. 64mm can be bored into the stock liner but it's paper thin. I used to buy damaged CB750 barrels as a source of cheap liners and had them machined to suit CB77 barrels. Crankcase mouth has to be opened up to take the bigger liner though.
And the stock head has that aweful step to clear teh tall 77 pistons compared to the shorter 72 pistons, but you can safely machine a lot of metal off that lower head surface. I used to get mine machined with an angled squish band and had a squish band machined into the crown of the pistons. That was a useful modification.
Patrick is correct that a single 26mm carb is barely enough. Arguably each cylinder breathes and then there's time for the other side to do it's thing. On a 180 crank they overlap so I'd keep the stock crank and use a single 28mm Mikuni.
Keep in mind that a CB92 (125cc mini Benly/Dream) had a single carb and I think they did around about the ton and were raced that way. I had a couple of CB/CA92 race projects including one with a twin carb conversion and I would stick with a single carb if I were doing this.
Just as a point of interest, mikuni say a 30mm carb has a 30mm bore. On a Keihn of that era, teh slide diameter is the size they quote, but the bores are oval (figure 8 style) and with a little clean up are actually close to the stated size. I like teh way that the oval bore is like a smaller carb at small throttle openings and like a larger carb when wide open. Very clever design for the late 50's. We used CB75o carbs on our CB77 racers and they are more or less 28mm when the bores are cleaned up.
And make sure the rolling chassis is up to the task. That would be fun to see the faces of other riders as you squirt past them.
Nice bike BTW, and I like your sort of insanity. Don't forget to put green O rings in that red circle