I strongly vote for the 6 speed. Basically this is your standard 4 speed with 2 overdrives!

I have one in my 34 Ford with 4.11 gears. Fourth gear is the standard 1:1 while 5th is .75 and 6th is .49......

Simply put, 6th gear cuts my RPMs over half (.49 reduction) of 4th gear. Which means with a 29.5" rear tire, I am turning 2300 RPMs at 100 mph! Talk about a highway cruiser...and it still has the 4 speed for city driving.

Denny is correct about the OD units...we are putting one of Gear Vendors units in my brother's 46 Ford and it is really cool. But it is also long, heavy, and (in my opinion) very pricey ($2500 just for the OD...you still have to buy your tranny). So considering you will probably not try to break any speed records at Bonneville, I would bet an F-body 6 speed out of a late model TA or Camaro (or even a vette if you find one) would solve your problems for the least amount of $.

Just my opinion...your getting some great advice from these guys.