Hot Rods rear axle

i have a 39 ford pickup and going to change the rear axle as i am in the
process of putting in a 302.when i measure the width am i measuring from the back of either side of the backing plate?i need to find a suitable donor axle if anybody can help suggest a suitable replacement.its pretty much a standard 302 so not producing alot of hp.

 

I find it much more useful to measure from wheel mounting surface of drum to wheel mounting surface so track can be matched exactly.

 

embyman got it right.If you use the 1946-47 3" wider bed, you can fit a wide range of 9" Ford pickup axles. I have driven my 39 every day for 17 years

 

A 8.8 out of an Explorer should fit fine. You'll have several gear choices, trac-lock and disc brakes. They are easy to find and prices are cheap.

 

8.8 out of a ford ranger pickup is perfect and cheap. Also plenty strong for a V8.