Magnum 300 crate engine - how should it go when tuned up right?

Well I suspected. And it's slower than a 318 2bbl? That has to take some WORK

Mufflers.......not a chance, unless they were for a Pinto. You obviously have duals, and there might be "a little more" there but nothing like you are looking for

Distributor........You know the ignition curve? That is where you START. I've seen "everything" in my 70 years including RUSTED ADVANCE mechanisms which were just plain LOCKED

WHERE is the timing initially and HOW FAR does it advance? I DO NOT EVER play with an unknown engine WITHOUT using a positive piston stop to verify TDC and then grabbing the timing light.

Carb........You say you "need a rebuild." Does it leak? Won't idle? Why? "Needs to be leaner?" Why? At what RPM? Idle/ mid/ power? You have to figure out how to nail this down.

FRANKLY IN THIS DAY AND AGE I don't know how anyone can justify buying an expensive engine and NOT springing for a wideband O2 setup. They just make things SO much easier

..........And then there's the "stupid simple" stuff.........have you checked that the carb is actually fully opening with the throttle?