360 questions
I echo the same "Keep the LA block" thinking. The roller cam in the Magnum is a broom stick. The LA heads even if there not a X or J can be redone & ported cheaper than new Eselbrock heads and there trip to the machinist. Though getting them to flow as good as Edelbrocks head is a different story often requiring a higher price tag. The amount of flow the letter head cam get can still make for a very powerful street/strip engine.
Cam wise, a stout Hyd. Cam or a really beefy solid grind is still cheaper than a roller cam. Excellent performance can be rung out of it easy enough with careful tuning.
To help keep within your budget, search here and elsewhere on line for some of the parts you require. If your handy on rebuilding a carb, that will save you an easy 50% on the price vs a new one.
I don't know what kind of build your after. But I have run 12's with stock port heads. Though the car was kind of narrow in its street performance manors.