I just bought a 48 Backhoe for my JD 5303. Based on my research I wanted to buy a used hoe with the correct mounting system. The factory setup for the 5003, 5005, and 5E series mount the BH to a long subframe. There are 2 heavy brackets that bolt to the tractor. The front to the clutch housing and the rear to the PTO housing. The BH and subframe assembly is then mounted to these brackets. The subframe provides for a strong rigid mount and distributes digging loads away from the transmission casing.
When I first started looking for a 48 BH, I found one on Ebay and bid on it. I quit bidding at $1800 and lost it at the end. This was probably fortunate because to purchase the subframe, brackets and hydraulics from JD would have been at least $2500. Also, the BH was in Florida and I would have had to drive there to pick it up.
I ended up buying a 2 year old 48 BH that had been removed from a 5E series JD. It came with the subframe, brackets and hardware. I am in the process of installing the BH. It looks like it will bolt up without too much trouble. Check your S/N. The later 5003 series use the same brackets as the 5E series.
I am sure that there are other ways to mount the BH, the factory setup looked like the best for me. I have read on the net of users who have cracked the transmission housing by mounting a heavy BH on the 3 pt hitch.
Hope this helps.