The BEST CHEAP rear end for a Model A is a V8 Maverick. 8" Ford gears and it is 56" wide.
Just be aware that POSI's are very difficult to come by.
Next is a 57 - 59 Ford car rear end.
Check this out:
<a href="http://hiqties.hypermart.net/9inch.html" target="_blank">http://hiqties.hypermart.net/9inch.html</a>