you'll need a flywheel specific to the 70's era 360, it's different than the magnum.
make sure that the crank on the van motor is drilled for a pilot bearing
you *might* get away with the radiator I/O location on the hoses, but do take note of it. especially the upper might be very close if you're using big diameter clutch fan from the van.
i loathe wheel adapters, so you're on your own there. 100% the axle will have to be narrowed. to me, unless you're getting a smoking deal on that work there's better options out back that provide many, better, positive outcomes: same bolt pattern, weight, serviceability, parts availability. but that's up to you to decide.
exhaust: dougs or TTI fit the best but are also the most expensive. pay less and wrestle a basket of snakes if you roll that way. if you have smog ports you'll need to address that issue by plugging them if the header flange & gasket don't seal that area (however it's good practice to plug them even so).