I can understand why you want to keep it original, and that's a good idea. How about this- - - -find a 261 engine, and do some go-fast mods on it, and keep the 235 in long-term storage to go back to original if you ever decide to do that. The 261 is almost a bolt-in swap, no permanent modifications, and it responds well to upgrades like overbore, stroke increases, camshaft changes, increased compression, and even multiple carbs- - - -IF you do your homework on venturi size. Most people go too far with carburetion and end up with something that's only good for wide-open running. A relatively short-duration, high-lift camshaft makes good low-end torque, but you'll end up with piston-to-valve clearance problems unless you do some creative machining. The "848" part number cylinder head for a late-model 235 works well on a 261, giving a compression increase. That's particularly true if you increase the bore and/or stroke. Do your modifying on a replacement engine, and you can keep the original one on reserve for a retro-fit later if you want to.
Jerry
