well, the intake manifold fits on all engine sizes from 262 to 350 and is also good for the 265cui engine.....
allthough I have to say, that the smallest V8 engine Chevrolet has ever built ist not the best engine to tune to more power.....
it´s more difficult.....
the reasons are caused in the size and the bore of the engine....
It has been designed for a low cfm, which means smaller carburators......
Your engine came from a small 2 Barrel ( with aprociatmately 235cfm and even the 4bbl had only 365cfm) and was now upgrated to a 500cfm, which is almost double of the original size.....
so many people would say, that one "pumps" a lot of more fuel into an engine, which is actually not able to burn it....
If the fine tuning is not right, one runs into problems.....
So to make sure, that the engine is still ok, I would suggest to go back to the original set up with the 2bbl carb and intake manifold....
anyhow, if you still want to run the bigger carburator :
1. check the spark plugs....if they are wet after you have started, you still have to much fuel running into the manifold
2. usually the carburator is not preset to a certain terrain and temperatur, so not for your special area...so check : does the choke flap is completely closed before you start the engine ( the choke flap is that piece of metal on the top of the rear part of the carburator).
3. Check whether the manual choke is operating right or if you have an electric choke, that the wires are connected right and that the flaps are closed when the engine is cold....
4. Do you have closed ALL little tubes in the front and the big one in the rear with a fitting or a rubber cover? if your carburator get additional air from there, the engine will never run properly.
5. check the two screws in the front.....turn them in a little bit.....
try again......
and as you can imagine : it is not easy from the distance to tell, what is wrong.....
so good luck then....we all have gone through problems like that...more or less...