I had a 53 Ford flathead with a marine transmission on it in my ski boat. It used the Mercury bell housing and the transmission bolted right up to the flywheel end. The flywheel stayed on and there was a stub shaft that bolted onto the same bolts as the flywheel used to bolt to the crankshaft. Most factory marine engines are mounted in the reverse position, meaning that the flyhweel is on the forward end of the engines. The flathead does not lend itself to mounting this way, so they are mounted with the flywheel on the back end where the reverse gear is mounted. The pan was standard issue. The exhaust manifolds are water cooled by an exterior pump or pumps in the case of the V8. The water is then exited into the the exhaust pipe after a goose neck that prevents backflow of the water into the engine. If you need parts to build one, there is a company in Hemmings "Not Auto Related" section that buys and sells old marine engines and they may carry some of these parts. I have seen flathead marine engines for sale in Hemmings.