1CM and EAB heads give you the highest compression ratios so you may find a few hp in finding the highest compression factory heads you can. http://groups.msn.com/FordFlathead1932to1953/general.msnw . .
I don't think you will notice any difference and do a lot of work for nothing. Most flatties don't show improvements without cam, carb and head upgrades. Then there's bore and stroke. If it's stock just drive it.
The cheapest way to improve to a stock flatty is to flush the daylights out of it to get all of the casting sand out if it's not been done already. You'd be surprised how much sand is left in them.
Mercury heads are the lowest compression - EAB heads are the highest stock compression. Don't do it unless you're about to slap a blower on...and even then...