It’s my fun beater run about car.
I’m trying to finish up my engine build for my 75 TA.
it’s the first engine I’ve completely assembled from scratch
I’ve helped do portions of builds but never the entire engine.
OK, knowing that, I chose to run the magnum heads, and not worry about the cracking valve seats. They perform better for street use, 1.6 ratio rocker arms and, IIRC, a slight bump in compression with better ports, too. And then if you're out running around, it's easier if you need parts on the side of the road or something.
It's all from the 'same' family and to me, that keeps it simple. The crosswind intake made it easy and I just ran everything on the front from the Magnum. I used a van PS mount to move the PS pump down below the factory battery tray.
Magnum heads on LA is easy. Oil-through pushrods and lifters (and the lifters are usually already there) and done.
LA heads on Magnum is a bit more challenging like others said. Oil has to be routed to the rocker shafts, either via block mods, or other means. I'm not sure how the aftermarket aluminum heads are.
That said, the few engines I've seen from the transition era have the galley drilled, whether Magnum or LA. So far as I know, you can keep the roller lifters with an LA head (Late LA engines are roller lifters) but others may chime in with more experience than me. So, it's probably worth looking at what you have before you make a hard decision.
Hopefully others will be along to review, it's been awhile since I did all this to my car.