The 5.7L are way more common - the 6.4L is surprisingly scarce given how many they have put into cars, trucks, etc.
The 5.7 is a very compitent engine - 375hp but there is plenty of room for a cam swap, headers, and getting to 500hp with very little work. The 6.4 is what? 485hp from the jump -
The T-56 will bolt up behind the 5.7 or 6.4 with a SBM to T-56 bellhousing. They use the same flywheel as well.
