What is original? Your "original" '48 1/2T rims should have been 6 lug 16". . . Seems like they got replaced. 215.85R16 would be the 16" tire. Someone else will need to help you with 15" tires.
I too am looking at 15" tires on my '51 1/2 ton. I have the six lug 15" rims, can someone recommend a stock looking tire size in radials?