found an 8 3/4, what's it out of?

FYI, before someone else mentions it, that chart is about as wrong as wrong can be on some of those 8 3/4's. You'll notice it lists all B rears as being the same width, they vary quite a bit from year to year, even from 62-70.

As far as a 56" backing plate to backing plate measurement, I'd guess C body. Especially given the info that he was pulling them for hot rods, because C-bodies are a lot easier to find in the yards.

These are flange to flange measurements, but you see what I'm getting at...

Housing widths, flange to flange

A BODY
'66-'72= 52 5/8"

B BODY
'62-'63= 53 1/4" (And '64 Max Wedge)
'64 = 55 5/8" (Exc. Max Wedge)
'65-'67= 54 1/4"
'68-'70= 54 15/16"
'71-'74= 57 7/8"
'71-'73 wagon= 59 7/16"

C BODY
'65-'69= 56 3/4"
'70-'71= 57 7/8" (Chrysler & Fury)
'70-'74= 59 7/16" (and '69 wagon)

IMPERIAL
'65-'66= 57"
'67-'69= 57 3/8"
'70-'71= 54 3/4"
'72-'73= 59 5/8"

E BODY
'70-'74= 56 31/64"

A100
'65-'70= 56 3/4"

D100
'65-'71= 58 5/16"
'72-'74= 59 7/16"

Check the perch widths too, if its 46" c-c on the perches its either C or E. B body would be 44" 62-70, 47.3" for 70-74.