The 5-speed can be fitted to the 350 or the 500, but you have to convert the gearbox to right side shifting, because the left side shifter rod needs the tunnel thru the engine case, which the 4-speed models do not have. There is a kit available to change the 5-speed to right side shift, and left side brake pedal.
The top end from a 500 could be put on a 350 engine case, but there is some welding and machine work involved to enlarge the opening in the case for the larger barrel, and the stud locations must be moved outward for the 500 top end to fit.
The 350 has a slightly narrower con-rod at the small end, so it isn't really suitable for performance work on a 500 top end. But it would work in a typical low performance application, and the 500 con-rods including the Carrillo rod can be put on the 350 flywheels if you want to do a performance build.