Here is the thing. The 235 responds to engine modifications just like any other motor. More compression, more carb, more cam, more ignition, all adds up to more power but its relative. Little motor little gains. That, and the design of the motor essentially limits how much improvement you can make. Early hot rodders learned this early on and that fact begot the famed 12 port heads.
I have seen inquiries, "how do I make a 235 or a 261 run like a raped ape." Well the simple fact is you cant. But if you like to tinker, and I do, you can do all kinds of things to modify a 235. You just have to be prepared for the old adage for every action there is an equal reaction.
My motors are all modified. Dual and triple intakes, of my current Clifford intake with a pair of Holley 350 2 bbls. Vertex magnetos, solid lifter cams with big lifts and duration, forged pistons, GMC rods, all kinds of stuff. You could build a V8 chevy cheaper then my motor and it would go way way faster. But I like to tinker. If you don't like to tinker, then modifying a 235 or 261 or any inline is not for you. If you like dead reliability, stick with stock. The engineers designed a very reliable stock motor. No reason to mess with perfection if you want no hassles.
So, if you like the looks of a dual exhaust, and want to keep the stock heat riser system, you have to make a dual exhaust manifold out of a stock manifold (unless you are lucky enough to come by the rare 53-55 vette exhaust manifold). There are a couple of ways to do this. One, simply cut a hole in an appropriate spot and braze in an additional outlet. The old catalogs, like JC Whitney use to sell add on outlets for just that purpose. This would be the quietest version.
The second way is to actually split the manifold, and braze in a sheet metal baffle with a say 1 inch hole in the baffle, add an additional outlet on the other side of the baffle, and then weld the halves together. The bigger the hole, the bigger the rap effect, so a little bit rappier. The third way is to isolate the manifolds. Cut in two, braze the ends closed, add an outlet in the second half and away you go. This would be the rappiest version. But all would allow you to keep the stock heat system to the carb.
The Fentons save you the fabrication issues (yep, rusty carbon soaked cast iron has its welding issues) but you loose the heat to the intake. You can use the plate and copper tube arrangement for heat, or run hot water to the bottom of the intake. I don't want to opine on which is better. I use hot water. The one exception, Fenton at one time made a dual exhaust with the heat riser. The are very rare. I have never seen one in person, only a photo.
There is also the cheap but crappy sort of tube headers. Those are not worth messing with but if you do, same issues as the Fentons.
The best tube headers on the market are the Clifford Headers. But they only sell the shorty version now as the jigs for the long tube versions were either destroyed or otherwise disposed of during a dust up between some person who was working for Jack and his family after his death. But the Clifford headers, long or short, suffer the same issues as the fentons and crappy tube style. So you need hot water or hot air.
