Before you buy any parts for this one make sure it will physically fit in that 66 Barracuda. Maybe do a test install. I do not know that answer, but maybe somebody tell us if they used one of these in an early A body.
It can fit, if ya make a couple of hammer strikes. Put a 727 behind the 408 in my son's 65. Had to smash the firewall/floor seam flat and clearance the floor for the shift and throttle pressure levers. Others have not had too, so maybe it was the original mounts we used, but we also had to "mini tub" the trans tunnel about an inch and a half.