The stock H distributor is a vacuum advance only distributor designed to run with venturi vacuum from the carb. If you run multiple carbs it makes getting the spark curve correct difficult. Throw in a cam with overlap and things get downright messy. The G distributor uses a mechanical advance with a vacuum brake run off manifold vacuum (in low vacuum situations the brake retards the mechanical advance, but the brake is adjustable). You have to use the G cam and timing cover with the G distributor as the distributor bolts onto the timing conver and is driven directly off the cam.
The 97s and 81s are definitely expensive. What should be cheeper if you can find it is the Chandler Groves / Holley / Ford carb from the 60 horse. I know these are hard to find, so it may not be an option. Two carbs are probably a better idea than three anyway.
I don't think you can run flathead eight pistons in the six, the piston height, piston pin height, etc. is probably different. Besides 3/8 flathead pistons are only 3.375, .080 over H pistons are 3.380
Neither the H or M had liners.