Depending on year of 5th avenue, either 7.25 or 8.25. sometimes you have to drill a matching 5th hole at the axle flange mount as these lighter duty rear ends used a 4 bolt mount instead of the 8.75's 5 bolt flange. Same pattern, just missing a hole. You can use any backing plate as long as it's for the 10 X 2.5 brakes. Other donors could be 1/2 ton rear wheel drive dodge trucks and vans, volare, cordoba, etc.