In 91( I assume it's an automatic because they never offered a Monte SS with a stick) they used a 4t60-E (rpo code M13) exclusively behind the 3.4 DOHC. They could have used a or a 4t60 (ME9) non-electronic 4 speed or a TH125 (MD9) 3 speed auto behind the 3.1. A trans out of a newer (97-up) Monte SS will not work. The trans in your car uses a vacuum modulator for shift control, where the 97-up 4T65-E is fully electronic. Your car's computer doesn't have the ability to control the shift points on a newer trans, so you'll have to stick with one from a similar year car (pre-96). You'll also have to be sure your replacement trans has the same final drive ratio or your speedometer will be off.