Could you fit the whole rear axle from a 2500 in there?
Yes. A 9.5 14 bolt will fit. Some 1500's came with them factory. They are 6 lug. You can also find the 9.5 in a 8 lug fashion, which can be converted down to a 6 lug.
Also, I was reading a thread on another forum about converting a 10.5 14 bolt (full float) down to 6 lugs. That might cost more than finding a SF however. You will need a different u-joint either way. And some other small odd's and ends.
This is if you find a 14b out of a OBS truck. NBS, NNBS or OOBS would need some modifications however but very do-able