The 413-1 (1971) has 1" taller heads that flow water directly from the pump around the exhaust valves, which are sodium and do not have seals (intentional so they are always lubricated - there was a TSB about it).
Reverse rotation cam due to the timing gears rather than chain.
7.5-1 compression, heavy cast exhaust.
Can be run WOT 24/7, as they were also on generator sets for the military.
I have seen my manifolds lightly glowing after 5 minutes WOT up mountains.