b2200 or b2600 trans swap into a b2000 ??

okay.. my dads 86 b2000 tranny suckss.

idk if mazda made different trans for these trucks but in this truck your goin 65mph at 3k rpms..hmmmm

well heres the point.. fron 4th gear to 5th gear theres barely any difference. about 150rpm drop

so we believe that the trans is like an in town geared or something.. any ideas?

and if so can i swap in a b2600 trans or a b2200 trans? ??

let me know because this is buggin me.. he drives on the highway daily and this doesnt help
this truck should sip the gas.. not chug it.