This sucks. I completely still went 4R. I can't take my eyes off of the MW9...
Ok, I think I have this figured out. What color/teeth count is your drive gear? Please TELL ME IT'S RED with 17 teeth!!!!!
If it is, you can get the purple gear with 43 teeth. GM p/n 1362196. Housing for the driven gear if you need it is GM p/n 25512339. Housing also fits TH350 and 7004R and is normally cast with the driven gear teeth numbers it can handle.
You are looking for gearing for 3.42 with 27" tires.
Speedo drive/driven gear calculator below. GM modern speedos use 1000 revs/mile @60mph of the speedometer cable.
http://www.bgsoflex.com/speedo1.html
Plug in 17 (red drive gear) and you get 43 teeth. = PURPLE with 43 teeth.
19 with the yellow drive gear and you need 49 teeth. Which ain't happening. Not without some sort of adapter gear.
Again, I make no promises on anything I try to do in order to help. It's free advice, but I'm limited in manuals and information.