408 question and advice please

Cool. That sounds like a good start. The main reason I like the magnum vs LA is the roller cam. I also like the magnum head design allows you to run a smaller quench which will help with lower octane fuel use without detonation. you shouldnt have any trouble at all especially with the aluminum heads. I would look at the KB745 pistons and possibly a Comp XR275 cam. With the other parts you have picked out that will make a VERY stout combo. I will go with the same pistons but probably an XR268 cam. I will be limited on valve lift because of the valve guide interference with the stock heads. The plan is to get the thing running and then buy the same heads you have and the 275 cam to liven it up the bit on down the road. You shouldnt have any trouble finding the block you're looking for- they made a pile of them. Just remember you''ll need a different timing cover and possibly a camshaft extension to run a mechanical fuel pump. Or just go electric.