Disappointed magnum head flow numbers....

I realize that I need good
exh flow that is why I put a nitrous grind in it.
I anticipate around 500 horse on the combo with 150 shot of nitrous I was hoping I could accomplish it on stock heads
I don’t have the money for aftermarket heads that would definitely do the job
I jus want an estimate nothing is chiseled in stone
360 mag
Stock block/ heads/ crank/rods/ 9:1 pistons
Edelbrock knock off airgap
MSD 6al my dizzy accel super coi904 w: 3500 converter
Massaged 750 dp
You already the specs on the cam engine is not that far off 380 crate

With a 150 shot 500 hp should not be difficult to hit with magnum heads.