Okay. Well that makes me feel better about my IRS. I guess I'll just have to look at their cars and see what they've done to the suspension. I just heard our axles go at 400 RWHP. Is that correct?
Like anything, you really mistreat the half shafts, they'll go.
As pointed out elsewhere, the biggest problem is really hard launches with a standard. The torque converter has enough give that you can feed an auto setup a lot more power.
A GIGANTIC suggestion - FIX THE RUBBER.
If you're running 20 year old rubber back there, then yes, a hard launch can damage everything. Which is no surprise ...
Again, DLF makes some nice Delrin parts. Energy Suspension has poly available (although you have to do some work to make it all fit - and it's not just "I have a 1997 TBird, what fits?" from the catalog. Search here and be educated, young padewan). There's even people making aluminum bushings, but for a street car, I'd HIGHLY recommend not on those.
Past that, find a 1997/1998 Mark VIII in the U-Pull-It and get the LCAs and half shafts out of it. There's articles here on what the LCAs take to fit and work properly, the half-shafts just fit in and work.
Don't be too cheap. I understand price shopping for X (if you pick Tokiko blues, then it's better to pay $200 than $500 for the pair for the back), but don't confuse Brand Xpesive parts cheap vis Brand Crap parts even cheaper ...
Do NOT buy just on price. Consider the reputation and quality of the company making the parts first. Then the rep etc. of the company SELLING the parts.
I'd go ahead and get a cast iron pumpkin built up with the gears you want - there's possibly someone local to you, or you can contact Alan (Dirtyd0g) and see what he wants to rebuild one with the gears you want. I mean, if you're redoing the rear, you'll end up with the half shafts out ANYWAY.
DO pay attention to the U-joints! I did greasable on my car, but I'm running the factory 200HP 5.0HO ... for your consideration, there's stronger and better U-joints to put in the driveshaft. DO IT. Don't argue, just do it.
While the shaft's out, if you drop the gas tank, redo the pump while you're there.
Don't forget the tranny mount and the two motor mounts before you start beefing up the motor any.
Last but not least, don't ignore the front end - yes, a wobbly front end won't toss the half shafts. But if you're laying 600HP down to the ground, you'll need to keep the front end pointed the right way ...