Jim III,
my 51 1 Ton has the original 17" split rims. They are listed in the GM data as 17 x5 with a 7/16" offset. I think that puts the backspacing at 2 15/16". Measuring the front side of the rim supports these measurements.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!