Old School vs Magnum on 86 Dodge W100

The Magnum's higher inherent compression ratio will make it a better engine (torquier for low RPM use). You can get there or better with the older LA318 with a piston swap, but you need to re-balance the crank too, so the $$ add up to around $500 to exceed the CR of the Magnum.

In the cam department: You don't need to be fussy over the oil used with the roller cam, like you do with the older flat tappet cam in the LA318. But for a performance cam, the flat tappet system will usually be cheaper than the roller cams. IDK where you are thinking of changing the cam from stock; but be aware that you can't change the cam a lot for the factory FI as the manifold pressure signals will get changed enough to where the factory FI will not operate well with much of a cam change.

FI will keep the oil a lot cleaner.

I prefer the older ones for other reasons (like the shaft rocker system), but I suspect for your son's use, which is not demanding, the Magnum seems like the better way to go. You have all the FI parts.

You'll want a high out alternator to run the FI system.