From what I remember, the 4L80 fits in our Montes without any modifications to the trans tunnel. I would imagine the 6L80 and above would definitely require custom engineering. I LS swapped my Monte about 6 years ago and felt I was somewhat late to the we're talking modern LT swaps!

-'87 Monte SS LS 2/4L65e Quick Perf. 9" w/disk brakes
-'89 Iroc Z28 Stock 5.7 Edelb. TES headers, Borla Exhaust90K
-'77 Trans Am 468(Stroked) Doug's Headers Flowmaster catback
B & M shift Kit 400 Turbo trans. Edel brock Performer
RPM Intake Sean Murphy built Rochester Carb. Restored
in '08. 3K