At the stock motor's power limit of about 425-450rwhp, the transmission will be fine if its in good shape and so will the rear. Its when ppl put them in cars with a lot of miles that problems crop up.
When I eventually get this stupid motor swapped in my 2000 it will still be a stocker pullout from the JY with 38k miles at $575 shipped to my door, stock trans, Exedy Mach 400 11" clutch, stock T45 and stock rear. I do no forsee any issues as far as driveline durability but if something does break then I suppose that's just an excuse to upgrade it lol.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.