3spd to 4spd OD trans

good idea chuck i may do that.. considering im wanting to keep the stock parts stock. imma have to find a used A904 then to "mod" for that wonder if the 80's 904 is different from the 76 in terms of size/weight. if i recall the 904 or its bellhousing in the duster is ALU. not for sure, i havent seen to much "detail" as to what it has. assuming my car is all stock i will have to get info on all the parts. i say assuming cause when i got the car some years back in highschool for $1400. the front hood is steel braced not ALU like i have been reading about.

you guys think i could then increase gas mileage more by installing a GV? as from when i recall driveing around 55-60 it sounded like the /6 was revving high, if i had to guess prolly in the 2.5-3k range.