My original lasted to 151k miles. I abused the car (daily driving, racing, towing) quite a bit back then. I changed the fluid a few times, about every 50k miles. It was rebuilt by AAMCO and reinstalled.
I removed the engine for rebuild at 246k miles, did not remove the transmission. I continued to abuse the car until around 260k miles; now I drive it fairly normally, not daily. Other than the TC lock-up clutch failing, the transmission has not given any signs that it might fail, now at 278k miles.
Things to consider:
Most parts on the Riviera will last ~150k miles, the engineered expected life of a '90s GM car. More mileage is possible with proper maintenance. There are exceptions, such as the 3800 engine, which should last 200-300k miles or more.
The 4T65E has gained a reputation for random failure. There is a chance it can go at anytime, or it could last another 50k miles. I wouldn't bet on it going much further than that.
You can remove/install the engine from the top. I found this to be quite easy compared to dropping the cradle, which is how AAMCO removed my trans. I don't know if the trans would come out the top side with the engine (may need to do them separately).
Much of the cost of rebuilding the trans is removal/installation. I believe it was around $1000 in labor back in 2007. If you have the means to remove it yourself, a rebuild at your mileage makes good sense, imo. If you do, I recommend replacing the TC clutch material. Also a good time to replace engine & transmission mounts.
Good luck. Let us know how it turns out.
'98 SC Riviera 268k miles 298 HP/370 TQ 0-60: 5.79s ET: 13.97 @ 99.28 4087 lb 20.1 avg MPG Nelson Ledges Lap: 1:30
3.4" pulley AL104 plugs 180Ί t-stat FWI w/K&N 1.9:1 rockers OR pushrods LS6 valve springs SLP headers ZZP fuel rails
KYB GR2 struts MaxAir shocks Addco sway bars UMI bushings GM STB Enkei 18" EV5s w/ Dunlop DZ101s F-body calipers
EBC bluestuff/Hawk HP plus SS lines Brembo slotted discs DHP tuned Aeroforce Hidden Hitch
'05 GTO 49k miles 0-60: 4.8s 16.9 avg MPG Nelson Ledges Lap: 1:26