Most likely the POS accl pump seal is swelled up and/or someone left out the return spring under the pump. My guess is that they used one of those new "blue" pump seals everyone sells. Those aren't worth two squirts of duck poop and swell up very quickly in this new fuel. I'd also add that using a carb from a 350 would have NOTHING to do with being down on power. The 455 carbs weren't really that much different in 1970, same 750cfm and PLENTY of carb for what you are doing. Another observation is installing the 212 cam and not upgrading the torque converter. That cam is a great choice for a mild 455 build but it shifts the power curve up quite a bit compared to a stock cam and isn't very happy with the factory low stall converters they used back then. A good converter would be a HUGE wake up call for what you are doing. I'd contact Jim Weise about a TSP unit, nothing else out there even comes close......IMHO....
Also - check your total timing. HEI's have traditionally a LOT of timing in them, which limits your ability to have reasonable initial timing, like 10-12 degrees. Before you do anyting else (except the accel pump), know and understand your timing curve.
I believe if you install a correct accelerator pump ( from Ken or Mark here on the board) and get with Jim Weise about a custom converter you’ll be burning rubber
