3 and 5 rib are 002.. 1st thru 4th gears are all the same, jus the RnP is different. 6 rib is 091.. the difference is an 091 has wider gears (hence the longer case)
all the parts in a 3 and 5 rib are interchangable
5 rib (4.86 RnP) is a good trans IMO.. will give you that little bit of speed needed to in first gear to climb
6 rib (4.56 RnP) is the way to go if you plan on building serious power later on down the road
you can pull and 3 rib and drop a 5 rib rite in, but a 6 rib requires a lil modification due to the fact that its about 3/4 in. or so longer than a 002 (3 or 5 rib)
they actually made an 091 3 rib with a 5.42 RnP, believe it came in a diesel.. all the ones i have seen where side shift
i ran a 5 rib for several years against a mild stroker motor and 36" cut buckshots.. the only mod i did to it was install an aftermarket 4 spider gear diff and reverse and it gave me zero issues
* had a buddy with a strong 1835 and 31" v-treads, ran a 3 rib and it was too slow, jus blow the tires out... he came up with a 6 rib and it wouldnt get out of its own way
.. point being, it takes POWER to run a 6 rib, especially from a dead stop on a hill. he coulda put a 5 rib in it and been good to go
BUT.. depending on budget, you can gear a 6 rib any way you like