Don just about said it all. I have built two flatheads and they were a 51 merc and a 53 merc. I bored them both out .040 and had no problems with overheating. Definitely go with the 4" merc crank and use 2 not 3 stromberg carbs. With some good heads and a 3/4 race cam and you shold get around 200 HP or more. As Don mentioned finding a good block thats not cracked is the problem.