If you are already planning to balance the assembly, yes you can interchange the crankshafts. The Magnum crankshaft takes the same bearings, timing chains, and harmonic balancers but the one advantage is the rear main seal area on the crankshaft is ground smooth to take the modern two-piece seals with less leakage. All LA and aftermarket crankshafts for LA engines I have seen still have the grooves cut in for the old rope seals, which nobody uses in LA or Magnum builds.
Connecting rods on Magnums are narrower on the small end than an LA rod and the pistons are narrower too, but if you run a Magnum piston in and LA you just have to run the Magnum rods, aftermarket rods that are narrow enough, or grind the small end of the LA rods to narrow them before balancing the assembly.
With that info, you can use whatever parts you want in either style engine by just making sure the compression height of the piston is correct for the block you are running, and make sure the rod is narrow enough on the small end for whatever piston you are running.
