Think RodStRace has given you some good advice.

Here is some inspiration. My 37 Plymouth coupe and the 230 flathead that I built for my 48 Plymouth coupe. It was my daily driver for about 5 years in Dallas traffic. The flatheads can be very reliable if in good condition.

Yours should be a 201 (82 HP). A 218 (95 HP) from a late 40s or a 230 (132 to 138 HP) from a mid to late 50s would be allmost a bolt in for you with the right bellhousing and starter. Either would offer a good boost in performance. I know when I went from my original 218 to the 230 there was a big seat of the pants improvement. Another think I would suggest is a Borg Warner R6 overdrive transmission. I put an R10 in my 48 and it was the single best modification I made to the car.