360 Magnum building on an existing bottom end

At a bare minimum, check the bearings- you'll have to replace the pan and pickup anyways.
Do a compression/leakdown test before you tear down the engine to verify ring seal.
Under 100k miles is nice, but doesn't speak to how the engine was treated during those miles.
Make sure the Offy tunnelram will fit your magnum heads- Magnum and LA intakes have different bolt angles.
Edit:
And remember that you can't use ANY LA (273,318,340 or 360) flywheel on a 5.9 Magnum. You need to either get an aftermarket flywheel or rebalance the LA flywheel to 5.9 specs. Lots of threads on this in the Magnum Swaps forum.
And make sure your crank is prepped for a pilot bushing/bearing.