Can a 1991 4 cylinder engine be swapped out with a 6 cylinder on a 1991 Mazda B2200 5 speed RWD?

It is interchangeable for sure.However you would have to change the wiring harness out from your current that feeds the ecm.That would mean removing the current ecm and harness in the truck and place the harness and ecm from the v6 more than likely.I am not sure if the harness for dash side going thru the firewall would or would not being same(likely would need harness from the donor trk).Being the 4 cydlr is somewhat different than the v6 the wiring on the dash instrument cluster maybe the same and the fuse box may be different as well.I would suggest checking out mazdatruckin.com a member who post here alot runs that and would be better able to answer all the specifics bc there are alot I simply cannot not answer with exact knowledge of.It can be done,just not sure of the changes necessary being the 86-93 are essentially the same truck aside from models,eng,trans, and 2 or 4wd etc.Hope this helps.