New to me Barracuda with Mopar Crate 390hp low oil pressure

The R/T heads came with 2.02 Intakes and the R/T logo would be right under the 1 and 8 exhaust port, which is probably covered up by your hedder flange. if that's what you have.
The Magnum Head exhaust flow isn't the best, that's why Mopar suggests using big 1 7/8" hedders. There is a big difference in using a maximum flow muffler, like a dynomax ultra flow, and a turbo type or anything else with less flow.
It's like the motor is fighting itself, and it is. You don't want to smother a Magnum, it has it hard enough. Just the idle, is so much better when it's free.