Quicktime makes a bellhousing that'll mate the 03/4 Cobra and also the Viper T-56s to a small block Ford. Be weary though, since you've got a '66, you'll have to cut your trans tunnel.
I purchased a T56 designed for Windsor engines. Not cheap but you know what you are getting. It has the correct bellhousing and, mechanical speedo directly from Tremec.
I'm using a reinforced T-56 with the Quicktime bellhousing on my 69. Still cutting the tunnel even though its wider than the eirlier years. Why not just get the T-5 rebuilt and reinforced? Call Pro-Motion Powertrain for ex. ProMotion Powertrain
Centered a Viper T56 behind a Ford T5 blow proof bellhusing, which has more surface area to mate the tranny behind that a stock T5 bell. Mated a Ford bronze pilot bushing and a standard Mopar pilot bearing. A Spec stage 3 clutch for Ford TKO already came 26 spline and matched the Viper T56. Also used a Foxbody double hump tranny crossmember, modded for exhaust clearance. Since I used the T5 bell I was able to keep the cable clutch setup from the T5 I had. Whatever T56 you use you will need a bellhousing with correct depth for your input shaft and a a slip yoke to match. A factory Viper T56 doesn't have any speedo equipment and the shifter is different from other T56s. The shifter location is about the same as where my T5 shifter was located.
This pic shows the under brace still intact, I did cut the visible portion and widened the clearance to allow theT56 to tuck into the tunnel better. The hump was welded back on about 1.5"-2", and my engine was lowered about 1". Pinion angle at the tranny is about -1 deg.