I am a newbie to this site but have been building a  300SL replica for years. I spent a lot of time researching
the Factory Five Forum. Ultimately decided on the T-bird IRS over the Jag. I liked the look of the Jag better but in my application you just cant see any of it anyway. I have been reading all of the T-bird posts on this forum. I havn't come across anyone talking about the strength if the T-bird IRS. I built mine initially using a cast iron T-bird unit. I then got an aluminum pumpkin from a Lincoln. Swapped all of the guts into the aluminum unit. Easy to find a set of 3.27 gears with trac loc in the T-bird SC but the better gears don't exist in the Lincoln. Their are some who swear that they made Lincolns with trac loc but you would have to search too many salvage yards to find one IMHO. Anyway In my searchs I did not find any meantion of different lengths of axles. As far as I know they are all the same. I did find that they are different diameters though. Don't quote me but the 7.5  rear use a 1", the 8.8 uses a 1.2" and the Cobra uses a 1.5". So I scored a set of Cobra axles on E-bay and had them shortened re-spine then heat treated. I read a few posts on this forum about cutting and welding them together. The prevailing attitude seemed to be "so what if they break they will spin harmlessly and won't kill you". Seems like doing it right the first time is a better answer. Not sure I would want to be left stranded somewhere. The tow bill might exceed the cost of the heat treating.
Getting back on point I was wondering what the thoughts were on how much torque I can put through on of these units? If I beefed up the rear cover with a brace then as I see it the stub shafts coming out of the diff becomes the weak point.