72 chevy swb rearend measurement??

[rex01atv]

I'm trying to figure out how much to narrow my rearend, so i measured from backing plate to backing plate and came up with 59''??? Does this sound correct to anybody??? I want to run a 20x10 with 4'' backspacing with a 295/40/20. I already mini tubbed the wheel wells. I'm thinking i need to narrow rearend 1.50 on each side??? I have 18x9 with 4.50 backspacing and when i air the bags out it tucks perfectly! Can anybody help.