the old flatie mercs had what was known as a pisspot holly 4 barrel it was tall and small square on the base .
i have done several includeing my own 53 that i just sold . i did rebuild the motor . went to .060 over . droped in a 49 to 53 merc crank adding 1/4 inch to the stroke of the 239 the pistons were for the merc motor with a relocated wrist pin location. you wind uo with about 261 ci give or take a few 10's or so.
the camshaft i used a old iskie cam. offenhauser alloy heads which the mrec stroke and ob gives about 10 to 1 or abouts and a intake for 2x2 holleys . they dump down right over the intake ports and most have little or no problem setting them up.

http://www.btc-bci.com/~billben/flathead.htm and http://flatheaddrag.com/index.html and do a search for reds headers as red also has alot of parts around and si where i got all mine from good luck man a good running flatie has it's own sound that you will become proud of