Both LA and magnum engines have cast cranks. You would have to be well in excess of 500 HP to really worry about needing a forged crank.
People have completely transplanted a magnum engine and the factory EFI into other cars. It takes work and a comfort level working with the wiring. However, the stock magnum beer barrel intake creats hood clearance issues in a lot of vehicles and it is not really a perfromance piece. Better to use an aftermarket intake and EFI controller for the ease of tuning..
The exhaust ports are better than the LA exhaust port but the configuration is the same so manifolds and headers are interchangeable.
The rear of the engine is the same between an LA and magnum engine. Trannies are physically interchangeable between the two engines. The only issues is the late model computer controlled trannies require and engine controller to run them. An older 727 is a simple bolt on but if you are using a weighted troque convertor the weights need to be changed for the slightly different magnum balance.