"Would" is a derivative of "will" and has an implication of futurity to it even when used looking backward as in "If he'd turned when I told him, he never would have run into that policeman". Its futurity is dependent on a different point than "now".

To say "if he woulda" abrades the sensibilities because it contorts time.

It is perfectly all right to say "Oh!! I would have come along!" implying (...if only I had known where you were going), because that is simply a contracted form of the "future as it would have been from a past point" construction. But to say "If he would have...." meaning "If he had....." is as offputting as wearing your kidney on the outside.

A