Re: Mercury mark 75H $$$
If it's a REAL MK75H, it's a very valuable motor. These things were as rare as Hen's Teeth. I'm not so sharp with the ins & outs of authenticating an original, but I expect there's a few Old Timers on this site that have actually laid hands on a real one. And if I'm not mistaken, '57 was the only year they were produced.
Heck, even a "clone", if done well, would be worth picking up. Consider it to be like a "Hemi" clone, the prices of the real Hemi's are so outrageous that a nicely-done clone, though somewhat 'affordable', will still fetch a good price.
If you end up getting the motor, pls post some pics, it'd be cool to see.
Here's a couple of cool links:
http://forums.screamandfly.com/forums/showthread.php?p=1406312
http://www.alterscale.com/Mark_75H.html
HTH........ed
p.s. just to be sure we're not talking apples and oranges, the "H" model is the Very Limited Production racing version of the garden-variety MK75. If it's just a MK75, it's still a cool motor but value-wise much much less.