I am also thinking about replacing the LA engine in my race car with a magnum. What year junkyard motor did you use? What compression ratio are you runnung? Did you do anything to the short block? Would you do anything differently?
I wouldn't be in a hurry to go to a Magnum unless you can do it for less than a LA engine, which you already have. The RHS LA-X heads have the same, exact ports as their MA-X Magnum versions but are machined for the original style rocker shafts, intake manifold and valve covers. They also flow at least as good as the Iron Ram heads. You can bolt on the LA-X heads to what you already have whether you freshen the short block or not. Then work on the rest of the rest of the combo to work together - cam, converter, gears, suspension, carb tuning, etc. "Just" swapping engine types or parts doesn't always guarantee a faster car.