Just my opinion, I wouldn't bother modding a Mag block for LA heads. In fact, I'm not really a fan of Mag heads on a LA. Lots of good heads out there for each block style. Just sayin'
Either way, mag or LA, I'll be porting the cast heads. Mag heads in factory form flow good for the traditional small block. I do like the rocker shaft much better. I can port either.... I like the pinch point better on the LA 360.