Blueprint 408 (LA block) with Magnum heads.

Those are supposed to be 58 cc have you checked to see what they really are
what have you done to the valve gear- what rockers
you do not have a lot of choices in HR lifters
I suggest an inverse radius profile from Mike Jones- let him pick the lifters and check springs, let him pick the rocker ratios unless you have a lot of bucks tied up already
but get the rest of the combo worked out B4 dwelling on cam
get the forged pistons designed for 0.0 deck with a dish watch out for the difference between 360 and magnum decks can B below deck if you use the .028 gasket
measure twice cut once
good rods more important than crank but do both - depending on shot and boost
detonation will break anything - so did I mention tight quench
need to know the rest of the details trans, gears, weight, GOALS
you will not need shaft spacers (read all of this B3 Racing Engines LLC - Mopar Rocker Arm Geometry Tech)
but you still have to set your fulcrum height B4 ordering pushrods if roller tip rockers
see where the valve tip scrub line is and how wide on your current build
before you remove pistons measure deck clearance on all four corners and write it down - see if the block is square and to make it easier to order pistons if you know what you have