How much money do you want to spend and how close to the 50-60s rule must you be? Since you mentioned a T-5, being period correct must only apply to the motor or? There are repro HP heads available that are duplicates of the original go fast parts, you can go to three Webers etc. Going with a floor shift will help your ETs and 4 speed will be better than the 3 speed if period correct does matter. You can even add an overdrive to the back of a Saginaw 4 speed if you can find the pieces to make a good compromise.
If you go with the 55+ GM rear, you can have different pumpkins that can be swapped out easily for racing vs. crusing; the other choices require a lot more work to change gears.
You can also do a lot to reduce the car's weight, which is as good or better than adding power. It all depends on exactly what the "rules" that you two agree to are.
Worse case, make the day of the race some where in the 90s and require a 50 mile drive prior to racing. The flathead will overheat and you will win by default.
