They were also used a lot for industrial, military, pumps, street sweepers, ground generators at airports, stationery pumps/gens etc

On our race cars the pilot bearing fit in the flywheel hole and didn't go into the crank , I don't remember if that was stock or top fuel clutch thingie, but you might check that out before you have the crank drilled/machined.

Yes the size of the flathead and hemi take up about the same room except the hemi has a rear distributor --will be a less than one hour swap--have done them in the dragster in less, but was younger then