I'm making around 325 ft/lbs at the crank right now, and my T5 is hanging in just fine. As long as you're not running slicks or drag radials, you're probably not going to break the transmission very easily.
As for the nitrous shot, even though your '93 has cast pistons, it should be able to handle a 100hp shot just fine as long as the rings are in good shape, you don't have excessive carbon buildup in the cylinders, and you make sure to run colder plugs and use the appropriate amount of timing retard for the nitrous.
We can give you power estimates all day here, but like I'll tell anyone who ask for a hp/tq estimate, why not just do to a dyno, pay the $75 or less, and know for sure what it's making?
If it's not broke, you're not trying hard enough.
1983 Mustang GT T-Top.
New T5 installed, driving my 'Stang again.