Mothertrucker in AZ sells the Fenton headers at a good price.
The Fenton dual-intake works without modifications with the Fenton headers. You will have to heat the intake manifold. I use hot air with the Fenton intake (no matter what Langdon says) or hot-water with the Offenhauser intake (that intake, and the Clifford intake, if still available, have a side-to-side flow-through internal heat-chamber).
Patrick's is a good source for any parts, if he has them in stock (he will usually dicker on price).
Langdon's dual Carter-Weber carburetors are well-matched to a 235. Get his linkage kit, if you get those carbs.
One readily available (and relatively inexpensive) higher-lift cam would be a 261 cam (this was also used in 235 Corvette engines).
The stock mufflers are civilized-quiet and are a baffled straight-through design. I doubt if the back-pressure is different from other, louder mufflers. The dual exhaust is a must for higher performance with dual carbs (or, with a bigger, single carb).
