Wow, I missed an exciting thread while away at the PA Jeeps Show all weekend! �:D
After my cracked block experience recently, I doubt that I would ever try to squeeze that extra HP out of a 60+ year old flat head. Sure, the magazines in the 50's were showing what you could do on a relatively new / low mileage engine, but most of the blocks around these days are tired and old.
Why not go with the Buick V-6 conversion that has been popular for a number of years? It fits in there and can get you the HP you are looking for, for your overdrive set-up.
I think the hardcore "correct police" that you mentioned at the MVPA shows will tear you up on anything that wasn't a period �/ motor pool modification.
Regardless, have fun with your project and by all means, post some photos! �;)