I looked at this route as I am redoing the drive train in my 1964 GTO for the same reasons, more performance, better highway cruising and just plain MORE FUN!
I bought a Tremec TKO II (TR-3550) 5 speed, from
www.moderndriveline.com and mocked it up with a bare block to see physically where the stock shifter was oriented and how much "modifying” was necessary to the transmission tunnel and floor boards.
After removing the stock Tremec supplied shifter I ordered a Forte’s (
www.fortesparts.com) 67-69 billet Firebird shifter and bolted it in place (the location was perfect when I bolted up a S.S. replica of the factory shifter handle - Bob L's idea) and then made the decision to not use the Tremec.
The reason, too much cutting and modifying of the floorboards for my taste on a 60,000 original mileage numbers matching car. I am in the process of building a Muncie over drive transmission. It uses the basic 3 gear ratio’s and then instead of 4th gear being a 1:1 ratio it is 1:0.86. I plan to use this with a 3.55 or a 3.73 gear in the rear end once I determine the tire size. I see you are in Wisconsin, if you go the Tremec route and you are interested in the Forte’s shifter and a Hurst 5 speed white shift ball w/built in electric push button switch I will make you a super sweet deal (parts brand new, $400.00, sell $200.00).
Hope this helps you out