I just bought a 78 mercury with a 351M I would love to find another 71 400 engine like I have in my F100 and toss it in but that's not likely to happen
I already know about changing my timing chain to 71 specs which I plan to do as soon as possible

what else can be done short of a major tear down and overhaul of the engine
because it only has 75 k on the clock

 

Duals, maybe a 4bbl (smallish), headers if you can stand them. Next cam, a higher lift, shorter duration, with new springs due to higher lift.

After that, it's time to go 400 with some compression and other parts as you'd like.

 

My M like the Eddy 1406 that's on it, as well as the Performer 400 intake.