There where certainly Allison options for the FT's. I missed out on the block adapter and flexplate for one several years ago, wanted to pick it up just for fun. I believe it's an SAE #2?
I believe with one of those adapters you should be able to bolt up a modern Allison 1000/2000 with factory parts.. With an aftermarket controller for the trans of course. Aftermarket support for the Allison is better than ever these days, for years Allison swaps where not worth the effort or even possible. And the only ones worth messing with are the 1000/2000 series. The 2000 (medium duty) series can be reworked into a 1000 to include things like a park brake and options for bolting up a transfer case, and all 6 gears.