LA 360 or 5.2 magnum

I have a 69 Dart(motorless 318 car) coming and I have a good '71 360 motor and a 91 Dodge pickup with a nice running 5.2 magnum.
I would use a 904 and 3.55 diff.
I'm just curious if there would be any performance difference as they are? would the 5.2 magnum flex plate clear the 904? I think the 5.2 is internally balanced but I don't know for sure.
I have to check the truck to see if it has an A518. maybe the 518 with 4.11 gears would provide more performance but still be a cruiser
This is to be sold to my son but since he buys without exchanging money- I don't want use what I don't have.
Any thoughts??