differential advice

i have a 1989 pickup. origionally had a 4 speed tranny in it. when the motor blew i put a rebuilt 22r and rebuilt 5 speed. swap worked out great and i recommend it to anyone that has the 4 speed. but now i want to regear the diff. from what i hear i have a 307?? my options for stock are 343 and 373(could be wrong, but thats why im posting this=] ) ive heard a 411 can fit. my question is what is the biggest i can go withou changing the rear axle and what gear is recommended? any help is greatly appretiated! thank you all for your time!