I've built several engines with 3 7/16 bore one for the track and ine for the street. The street engine was built over 30 years ago and is still running. I used 21Z rods on the stroked crank.
So far I have had two complete 3 7/16 bore by 4 inch stroke Flatheads never tried to run one. I still have one complete engine available everything is in a 59A block anyone interested send me a PM.
Ronnieroadster