The 318 and 360 magnums both have slightly shorter deck heights than the LAs did. TBI and carb had one deck height and the MPI magnum had the shorter one, I am not sure why they made one ity bity change like that

Main bearing diameter on ALL 318 polys, LA, and magnums are the same includeing the rear main. The differance in the magnum and earlier ones is the thickness of the cap, it was changed so you could use one pan for both motors.