Mike, im putting a 52 motor in a 48 frame (the earlier all welded type) and the only problem I've come up against is the front engine mounting plates...not quite figured out the problem yet as im not 100% sure what plates I have... but the gearbox mounts and lower engine mount line up fine.
You could keep the original number if it hasnt been realocated to another vehicle, but even if it has you should be able to get a age related plate no problem.
Have a look at this chaps A10
HERE...you will see he has hardtailed his plunger frame if you cant get hold of a rigid rear end....depends how original you want it to be.