R4-D4 Interchange

I'd like to get some facts or opinions on what can be swapped on these models.

I noticed that the main case casting number is the same on my R4 (sorry no s/n) and my D4 5T. Was the same case used thru the entire 6G Thirty, R4/D4 series into the D4C?

Also, I know that the outer sprocket bearing changed from the 4 bolt cap to the 6 bolt plate in the late 40's, but are the rest of the undercarriage components interchangeable?

Thanks in advance for your thots...

Just about everything but the pistons,head and fuel tank are interchangable. Same block same crank ect as RD4