Transmission Fit Options

From what I understand, only a 904 or 727 will fit into a '68 Barracuda without tunnel modification. Anyone know or tried an A500?

My research indicates it's from the same family, but not clear on ability to bolt to a /6 or dimensional retro-fit. Objective is to get a better highway cruiser (e.g. lower RPMs) for longer distance drives with better MPG to car events and 100-mile weekend drives with friends (thx Brandon).