You have the same situation I do. Well, almost - my 8BA has a cracked cylinder, but I've got a stock bore block sitting and waiting. In order to keep driving I installed my merc engine. When I get the backup engine running again, the merc comes out for reboring out to 3 5/16" which will give me 276 cid with room for another rebuild some day.

Flatheads can generally always be bored .125 over - that's right, 1/8" overbore. Some will go to 3 3/8" and a few will make 3 7/16". So +.030 is just getting started. You've got several more rebuilds there. Just remember if you use the spare merc crank you've got to use merc pistons with it.

I'm happy running the merc stock engine, but you gotta chase the HP dream.... right? I think you'll be happy with the stock merc if you go ahead and buy the headers, dual pipes and improved intake stuff (dual carbs or 4bbl 390 cfm) to allow it to breathe to the max. You'll use that stuff when you build your hot flathead anyway.

Ain't this stuff great!!??