Stock heads/rotating assembly, hot cam/vlave springs- how far can you go?

Do I understand correctly; you have an actual Mopar Performance 300 HP crate Magnum? Is that running in the car now? If so, how many miles on it?
If your goal is to do Trick Flow heads, don't bother with stop gap upgrades. It will cost more in the long run.
Mopar 380 crate Magnum camshaft would have performed less than stellar with that tight converter, but the 4:10 gears would have helped. I would not copy that camshaft for your build.
Mopar always gave shitty support for their parts. I never got to check the pistons in a 380 crate engine. There was some indications it may have had higher compression than the 300 HP version, but who knows.