budget street motor 360 or 383 for 71 duster

Ok if I go small block should I go with the magnum motor over my L.A.. also which motor would be better to build on say I wanted around 500 to 550 in a year or two down the road? Is there anything I would need to get the magnum to bolt up or is it a direct swap
Again, use what you have. Magnum stuff is good and many people use it with great success. Personally I like the LA stuff.