I've run a few clevelands over the years and read up a lot on these engines. Open chamber 4bl heads, strange, most of the 4bl heads I've ever seen were closed chamber yielding near 11:1 compression ratio with flat top pistons.
I've always heard and understood that the 4bl heads are too large ported / valved (2.19 I 1.72 E) for lower rpm power, but great for nascar and the like. Therefore I'd say the plates on both the intake and the exhaust would greatly improve the problem - which really boils down to port velocity.... if you make the port smaller with the same flow, the velocity will increase.
I would suspect the carburetor first and foremost. Those jets sound very large for a cam that size. Have you ever used a desk top dyno program? I'd suggest that highly to give you some idea of what you should be able to expect from this engine running optimally. Do you have access to an instrument to measure air / fuel ratio? If not, they are now available through places like summit for about $150 and are an invaluable tuning tool. I'd suggest an edlebrock carb for dependability and smooth running with no more worries of blown power valves and leaking float bowls.
What do your spark plugs look like after a good power run?
If they are sooty black you are clearly running rich as you suspect.
Optimize the carburetion, first before you go tearing things apart changing cams etc. Installing the exhaust plates should be relatively simple and only cost the price of exhaust gaskets and time.
Enjoy
