The T-bird rear is 56" flange-to-flange. Mustang 9" rears are 54" and 52", depending on the year. Not sure of the Shoebox width, but a '49 Merc is about 54".
The most accurate way to measure a rear end width is from the wheel mounting surface to wheel mounting surface. The reason is ... Different width brake drums ... affect the measurement if you measure backing plate to backing plate Most rodders with a 49/50/51 Ford shoebox use a 5 lug Maverick rear end. It measures 56.5 inches wide ( wheel mounting flange to wheel mounting flaange ) I believe the T-Bird is 60 plus inches wide ... measured WMS to WMS ... so it would be too wide. .
Easy to find good rear axle with different ratio options is what you need so I would go for a Ranger/Bronco ll 8.8 rears they are the correct width and some are posies!
That many guys use axle housing flange to axle housing flange dimensions DRIVES ME CRAZY...........how the hell did that habit get started? It is damn near useless information if space is limited and accuracy is needed. As stated, the drum width and backing plate offsets are equally important to know and ARE NOT taken into account by this method. Just blow your nose and get that idea out of your head! When answering the OP's question the only really useful dimension is wheel mounting surface to wheel mounting surface. And even that is only part of the answer because wheel width and offset and tire sidewall width have to be taken into account as well. Ray
Boy is this true. Found out the hard way on a Buick. Always measure from end of drum to end of drum.. not backing plates. Some brake drums can be much wider than originals.
Most 8" rears are getting hard to find and pricey one rear that is getting popular for 1949-56 Fords is the 8.8" Ford Rangers 1990-92 these are 56 1/2" drum to drum and can be found at many Pick-A-Parts for $200 and under many of these are also lockers and they also use the same bolt pattern as your Shoebox more info on these can be found here: http://therangerstation.com/tech_library/Ford-8_8-axle.shtml Be sure to check out the charts below it tells how to ID them for gear ratio and locker or non- locker.