First off I'm new to the site and seriously considering a 3rd gen for an inexpensive track car. I have a couple other projects going on right now (Wrangler build and a C3 Vette), but I'm not as familiar with the third gens. I've worked with most of the domestic axles, except for GM's 9 bolt and 10 bolt. I know that the axles are 28 spline from 90-92 and 26 for the rest. I know of the 9 bolt. An LSD is going in the rear no matter what I get. What sort of power will those rear ends hold up to? And is the 9 bolt worth it? Thanks

Welcome to the board. These rear ends are pretty weak from the factory. "how much it will hold up to"? depends on what tires your going to run and if you hook or not.

Gonna be a road race car, little to none on the drag strip, so no slicks. Not committed to a certain size, but DOT approved cause I want it to stay street legal.

In my opinion the 9 bolt is not worth the extra strength. It is stronger but it is also very difficult to locate parts plus once you do locate them they are always used, and bought "as is".

I wouldn't put no more than 350HP on either rearend. The 9 bolt is a good rear axle but like the other guy said finding parts is a B***H. Most good machine shops or even trans shops can get you the parts but they are almost always more expensive than the 10 bolt and there is very little gear choice for the 9 bolt. You only have like 3-4 choices for gears unlike most major rear axles where you can basically get any gear ratio you can think of. If I remember correctly the gear choices are 2.73, 3.23, 3.45, And they might make an aftermarket 3.73. The 3.45 rear gear is the only factory rear gear that gm didn't list an RPO code for also. Most likely because you could only find it in the 9 bolt and there weren't that many of them.

If you plan on road racing it, then you are good to go with a stock 9 or 10 bolt.
As far as the diff, I would put in a Torsen T2 or a T2R if you go with a 10bolt.
T2 and T2R are gear based LSD and the T2R has bias pre-load. Allows grip when one of the drive wheels loses traction or is in the air (T2 is sufficient for road racing, and the T2R is more suited to Auto-X since you end up doing really hard tight corners).
http://www.on-trackperformance.com/Torsent-2R.htm

A member here bought a T2R for his Formula,
https://www.thirdgen.org/forums/orga...kicks-***.html

Was planning on hp/tq numbers in the upper 300s. Still good to go with the stock axle for road racing? How about the tranny? Has to be manual, if I'm going to swap it out I'd consider a T56 or salvaging a ZF from a vette for the right price, if not a stronger T5

T56 would be my first choice, ZF requires a special bellhousing to fit into our 3rd gens and is expensive to rebuild (from what I read a few years ago since SLP used to the ZF in the Firehawk).
Rear should last ok as long as you don't powershift with really sticky tires or do high rpm clutch dumps with sticky tires.

How's the stock T5 out of a 305 car hold up?

It doesn't! Rated for 245 ft lbs. max. Either go with a T56 or with a TKO from tremec.