8 3/4 question

i see a lot of either questons 489 or 741 dif is one better then other
whats the differance ...do they both bolt in the housing ,are there even more #s
the one i got for my dart is a 489 that i want to change the gear ratio and put a clutch type sure lock
i think my 67 pu is a 741 ? when i was getting bearings think i had to know if early or late
not sure what the 65 pu has
thanks