8 3/4 u bolts what length???

Can anyone tell me which is the correct size for 8 3/4 u bolts??? I have heard 3x7 and 3x8"... I am switching out from a 7 1/4 to 8 3/4 Have the complete rear end with shock plates. just need u bolts and also looking for a posi center section w/3:55 or 3:91 gears.. Thanks