Recommended posi rear end?

ok i just wanna make sure im clear. i checked my rear end and its got 9 bolts on the differential. so if i buy a 8-1/4 differential i can hook it up to my current rear axel and be set? i mean, is the axel strong enough? it is, after all, for a slant six. also, with a new differential, will i need a new driveline to handle the new horses?