d20 and IH d300 are the only options without changing the output housing/shaft. if you are willing to do a complete tear down and switch the tailhousing/shaft. you can run a d300, np203, np205, atlas, klune etc.....
I hope by beefy engine you don't mean another IH boat anchor? Unless you have a lot of cash dumped into your IH 727 I would get the tranny the matches up to whatever mill you go with GM, mopar, ford.
If you are planning on running a beefy IH engine
well I would switch the output to take a 203/300 doubler.
My 'scout' currently has 345/727/203/300 but will soon have mopar 451/727/atlas and will weigh 200-300 pounds lighter