I'm collecting parts still for my '57 9". I was told that 65-66 mustang axles will work if its a small bearing unit? Mine is a large bearing but I found some large bearing 9" 31 spline axles for a 65-66 mustang. They should still work?
If they are for a 65/66, yes I would say so. The axle lengths should be the same.
My experience has only been with the small bearing units and for them, I was able to swap around. I've swapped the stock small bearing 65/66 axles into my 59 small bearing case before. Recently I updated to a 31 spline axle and just ordered the stock Mustang replacements (which were beefier and still retained the small bearing.) Perfect fit!