[Ronnieroadster]

Never heard of an oversize flathead lifter. I can only speak for myself I have never come across a flathead block that had a worn lifter bore. Have you compared the diameter of the lifters to any others just to determine if they might be a bit smaller in diameter? If I read your post correctly you mention some bores are tighter than others if its possible to try original lifters is that still the situation?
On some of my race flathead blocks I use bronze lifter bore bushings which are only a few thousands in diameter larger diameter than the original Ford lifter bore diameter. These new bushings have a smaller inside diameter for the now smaller diameter lifters. To fix a worn flathead lifter bore so the ID is now back to the stock diameter it would be extremely difficult to do in the block. The reaming operation needed to open the existing worn bore would require a larger diameter reamer. Due to the valve guide bore diameter a larger diameter reamer cannot be used for the bushing boring operation from the top. Also the operation needs to be extremely accurate so the lifter bore is still straight. One company BHJ makes tooling for lifter bore bushing operations as seen in V8COOPMANS post above the operation is rather simple to do with there tooling when there's nothing in the way above the bore.
Ronnieroadster