Loss of power in the upper RPMs is generally caused by leaning out or weak ignition. Loss of power in the upper RPMs with backfiring through the carb is generally caused by valve float(weak springs). To rule out not enough fuel, do a 1/4 run with a fuel gauge attached to the carb in visible view. A wideband O2 is ideal. To rule out ignition, close your plug gap to around .035 - .030 and note any change. If the ignition is weak, closing the gap will cause some improvement.
My best guess is valve springs. A new cam requires springs that meet the requirements of it's profile. If you don't have a valve spring pressure gauge there really isn't any way for you to check them. There are minimum seat and open pressures that you probably don't meet. If you're on a tight budget and don't wanna disassemble the motor to take the heads in to have the springs changed, you could just change the springs on the car yourself. All you need is a valve spring compressor bar and an air fitting for the spark plug hole(both found at most auto parts stores). All that would need to come off would be the valve covers and spark plugs. Not a hard job at all.
I can give you more details if you get to that point.
Good luck.
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