He's right about the sealing. I think the chamber design on the Magnum is better than the LA, but now you have sissy stud mounted rockers like a Ford, so to me the heads are a wash. They come with roller cams which is a positive. The block is a shorter deck height for more compression, so that's a positive. The Magnum exhaust manifolds are a better design, but then, most people building for more power throw the stock manifolds in the ditch, so you can call that a wash. Now, had Chrysler machined those blocks for a one piece rear main seal, like both Ford and Chevy did, then that would have been the deciding factor, IMO, but they did not. Last, but not least, there are more Magnums layin around now as good buildable cores than LA, so by sheer numbers, they have an advantage. I'll stop short of saying Magnums are "better" than the LA, but to me, they aren't trash, either. I guess it's all in what you want and a matter of personal opinion.
