Throw the GM P rods in the trash, no good performance engine builder will use them for the power levels you want, especially with nitrous. Get yourself a SCAT stock type rod, they are made of 4340 and come with ARP bolts. Cast crank is fine, get a SCAT here also.
I've done several similar 383 engines, 600+ hp level on the bottle. SCAT cast steel crank and SCAT Pro Stock stock type I-beam rods, forged pistons, good heads. None have failed, longest lived one is 7+ years old and gets hit with 185hp nitrous roughly every other weekend at the drags, must have had 60 bottles of nitrous run through it so far. Runs low 10's 1/4 mile in 3400lb car. It's been tore down once for refresh, everything looked so good it just got new gaskets and put right back together.