Road Runner Engineering states the following from their book Blown Flathead :

1948 murcury flathead (the mercury has a longer stroke than a ford) will produce 260 hp @ 5000 rpm and 305 ft lbs of touque @ 2500 with the following mods:

1) Dual exhaust and headers
2) Dual carbs or 390 cfm 4 bbl
3) 3/4 race steet cam and kit
4) bored .125 with 4 inch crank (again ford has a shorter stroke , 3.75 maybe?)
5) Aluminum hi comp heads with ( 63 cc)
6) 6-7 pounds of boost from the supercharge ( all you need to go with on that engine without forged internals)

If you are really serious about it give road runner engineering a call and they will set you up with what you want to know, or if you want to read about it order their book, its full of alot of usfull knowledge about the flatheads.