Suggestions on flathead choice

[ford38v8]

Sounds like you aren't overly concerned with originality, and you'll want a smooth transition without grief, I'll suggest a '37 -'38 21 stud, using you heads with water pumps integral, and block off plates for the front of the block. This engine will provide insert bearings, and compatibility with your bolt-ons, and is considered the best 21 stud block Ford produced. Your bonus is a visual that most guys will not detect as not original.