For the "KISS" principle, why bother adapting magnum heads to an earlier block? For what most folks will do, just get the whole motor and gain a nice roller cam in the bargain. Time, money and frustration saved in the majority of situations
It's a simple and easy way to to get a significant increase in power form a LA motor.
Using a complete magnum motor requires swaping a lot of parts before you can drop it in a older chassis. Certainly not hard but I would contend that on the "KISS" scale it would be equivalent to installing magnum heads on a LA motor. Plus the cost of entry will be much more to procure a magnum motor than a LA motor.