My understanding was pretty much all Fords before '48 can use a 56" width (flange to flange) for pretty much the same reasons stated (wagon width) I would say it could vary a bit as it could be made up in wheel offset. If you are going to narrow the axles, try to grab the widest possible, the reason is after the spline the axle reduces in width for about 3" so cutting off a axle by 3" will put you in the reduced diameter resulting in some expensive work, this only apply's if you are resplining your axles, if buying new no big deal. I used a large Harbor Freight pipe cutter to make sure the tubes where cut square (only thing I ever used it for)
Next thing I would say (my opinion only), just short of drag racing with slicks or circle track how many have ever really torn up a good rear end, has anybody?? Just curious. You should be fine with a 8", I recently saw a Top Fuel slingshot built in the 60's and they ran a 50's olds rearend, with something like 9 splines, true it never hooked up but this thing had 1500+ hp, I put a maverick 8" in my new rod and I haven't ever lost any sleep about whether its stout enough or not, true the 9" is stouter but still haven't ever heard of a 8" ever being torn up, and most non rodders swear its a 9", but the biggest benifit is the drop out 3rd member just like the 9" and the ability to get bolt on disc brakes etc. have fun Matthyj