Here is a set of new 31 spine axles intended for those who want to swap in a Ford 9" rear end into their Chevy/GM product. It has the standard-length axle as was found in Mustangs or intermediate Ford housings. The axles are small bearing so they would fit into a 1960s Ford or Mercury housing
I have seen these at Detroit Speed for $720 for the pair. Never installed but the bearings are apparently the big Ford bearings so they may need to be changed according to your application.