Many people describe the 383 as the “big block that thinks it’s a small block”…for good reason, the stroke is shorter than many small block V8s.
Still, the 284 deg, 0.484" lift MP cam isn’t THAT radical. Sure it will kill of some low end, but what you’re describing sounds far beyond that.
It could be simply a BAD T/converter (stator clutch slipping). It could be that that cam was installed quite retarded (was the phasing checked?) It could be a too-slow spark curve, even stuck mechanical advance weights, this is easily checked with a timing light. Another easy test is compression, if quite low this could add fuel to my cam-phasing suspicion (or the too-radical cam theory), this will also confirm (or not) that all holes are pulling their weight. I’d expect your engine, built correctly as described, to produce at least 120 PSI.
The FSM describes a way to test the cam phasing without disassembly. If you use the specs from the Hemi section, your numbers should check pretty close to that.
I’d ask you to test the actual stall RPM (stock was typically 2,500-2800), but a low result could point almost any of the possible problems).
The heavens aren’t gonna fix this, you need some Holmes-like detective work.