Just a question: did you ever talk to that shop in Dundee, Michigan about your flathead? That's all they do, and I can't remember the name of the shop for the life of me right now.
Anyway, since you're probably not looking for gas mileage in a car like this, why not go 429/460 and make it move with less effort? 4500 is heavy for a small block to push, IMHO, especially when you want it to keep pace with a car that's only 3000. They can be had for virtually the same cost and reliability. A single 4-bbl, dual-plane intake, 1500-5500 powerband cam, and a 9.0:1 C.R. should put you in the ballpark. Don't forget some decent heads!