since you have a 91, you'd be better off swapping it out with a 1354 manual, instead of your 1354 auto. (or 1350 I always get them confused). But if its working I wouldn't mess with it since you won't gain anything.
There may be a way to make an auto a manual, but you can find them for cheap if you look around. Even if you find a junk one you could swap the rear housing and manual shift parts. The motor and manual shift hook to different things.