A common big-bore wildcat back in the 1960s and '70s was the .458 American, a 2" .458 Win Mag case that was a kind of ".45-70 for bolt guns." Some were made on Pattern 14 Enfield actions, which were very cheap back then. A 1.8" version should work just as well.
Back issues of "Handloader" magazine should have old loading data, and some of those powders are still with us.
