Just some words of caution, before deciding on what you want, look long and hard before you proceed with the T56 rather than the TKO. The T56 is a super trans, but it's very large compared to other alternatives & when you put the driveline angles where they belong, the stock tunnel size & shape IS an issue on the '64 to '67. You might also want to check into getting all the hardware that you need with the alternatives & add up the totals.
For instance, if you go out & pick up a T56 somewhere reasonable, better get one with the adapter plate as part of the purchase as most of the transmission vendors won't sell the adapter plate (& special pilot bushing since the input shaft comes away from the motor by the thickness of the plate) separately so you can use it on a T56 you've picked up elsewhere.
If you plan to run tall gears (4.11, 4.56, etc.) the double overdrive is sweet (realizing that 1st & 2nd gears might be used just to get across an intersection in town), but if you plan to stay in the "3's" with the gear, you'll be making pretty low RPM's in 6th for your motor torque, unless you plan on cruising 85mph on the interstate.
Check out this calculator for some interesting info on RPM's in each gear, etc.:
http://www.geocities.com/z_design_studio/transmission_z28_6.html
Good Luck with whatever you choose.......Odie.