750 is perfect for that engine.
The only way a carb runs rich is by being over jetted.
Jet it down.
Learn the carb inside/out. Read voraciously on how to tune it.
Otherwise you will end up hating Holleys and never get the best performance you can from your engine.
Do you have the MSD running on points or full MSD ignition?
If you still have points, an effective and cheap mod is to pull the Duraspark distributor out of a 351m/400/460 engine at the boneyard and install it into your Cleveland. They never came with electronic and MSDs are pretty blatant copies of the Duraspark.
You just have to make sure the system gets full voltage and you get a good coil.
The stock ignition feed wire is a resistor and must be bypassed or replaced.
Good luck
Dave
Edit: Make sure you don't have a blown power valve.
This is a common problem on Holleys over 10-12 years old.
Since then Holley has added a PV protection circuit.
If your carb does not have one, save yourself some grief and add one.
Edit2: I see now that I thought you had a Holley.
You don't actually say. I got the impression from later posts.
If you don't have a Holley, disregard my post, then GET ONE.