Probably your best bet would be an adapter kit and a built Turbo 400. Only other choice would be to find an old Allison transmission and bell housing. I assume your Flxible is a Clipper with a 'straight-in' drivetrain. Clippers are not too heavy, a professionally built Turbo 400 ought to work. Just install a big cooler. Here is a source for an adapter. Don't get sticker-shock!:
https://transmissionadapters.com/collections/gmc