58 1/4" wide rear end. What backspacing?

[dhondagod]

Specs:

9" rear axle from a 77-ish Merc. Monarch, factory disk, measures 58 1/4" wide from wheel mount surface to wheel mount surface.

Planning on running TT2's 17x9.5" with a 275/40 or MAYBE a 285/40 tire.

I know with the stock rear axle at 60" wms-wms that 5.5" b/s is what I need, but putting the 9" in at 1-3/4" narrower I believe I need 4-1/2" b/s instead.

Should give a little deeper dish and the illusion of a wider tire/wheel package.

Sheet metal is stock 69 camaro, minor rolling of fender lip, stock tubs/suspension. Im not planning on cutting on our car so I just need somebody to verify my thinking is in-line with what I need. Planning on calling A/R and ordering first of next week and will run it past them as well. Was going to call yesterday but ran out of time. Thought Id run it past you guys in the meantime.

TIA


Chris