T56 is a good choice for a high horsepower car and a car with 4.11 or so gears.

I would seriously look at a Richmond 5 Speed (which is what I have) paired with about a 3.08 gear or even a little higher. or a TKO 500 or TKO 600 5 speed with a 3.55 or 3.73 gear.

These will all bolt up using a 26 spline clutch with a stock BOP bellhousing or better yet an SFI bellhousing.

The T56 is going to take some work to make it fit, tunnel clearances, crossmember, bellhousing modifications, etc. There was at one point a kit from Weir to fit one, but it was a good chunk of change - $500 or so if I remember correctly. Being that it's a 98 camaro, that's an LS1 T56, if you are getting it cheap enough, you can re sell it and make it worth your while. Usually the LT1 T56 transmissions are the ones that people use for the old school swaps because it has a shorter input shaft.

I would skip over the T5, it's not going to hold up long with a Pontiac's torque.

Also, with transmissions like this you need to take into account your rear axle ratio, are you planning on changing it? What available gearing options do you have? Etc. Each transmission/engine combo is going to have a sweet spot for the rear axle ratio.

-Paul