Help Decoding this 8 3/4

Hello, I recently picked up this 8 3/4 for a 66 Plymouth valiant convertible with the belief that it was for an A body. But these jam nut axles are throwing me of

i measured the spring perches and they are at 44" on center and the backing plate to backing plate are 57 1/8. and drum to drum it is 60".

any help besides the normal reference to what google says would be awesome.

I was thinking i can just change out the drums and axles for A body ones which i just found on offer up and move each perch in .5 to match the 43". Is this thought correct ?

Thanks, Btw