The continuous use of high power through a auto transmission is
what defeated auto-engine conversion guys in the boatbuilding world
many years ago. Even in the 50s and 60s, when transmissions were stout,
they couldn't get away with using them in a boat. The gears and
bearings would heat up and lose their strength and fail. I'm surprised
that more homebuilders don't know that.
I had a 283 in a 13 foot crackerbox-style boat, and the
transmission was a Warner Velvet-Drive marine unit. Forward was through
a massive clutch pack, no gears, and reverse had some impressive
gearing. The transmission was so heavy I could barely lift it, and it
had a substantial oil cooler to keep it happy.
A local guy tried an automatic trans from a car in his boat, and
had it fail in short order.

Dan