Wouldn't you figure it, it isn't a simple answer. Sorry. The stock driveshaft is 51 1/2 inches. I'm not certain that that is an original '66 driveshaft however. I may have accidently sold it Thnking it was my old Swinger shaft...... That shaft is small U-joint (741 case), 273/A833/8.75. BUT, it may have been from a '69 (742 case small yoke) 340/A833/8.75.
The driveshaft I am currently using is 53 1/8. That is large U-joint, stroker 340/A833/8.75. Here is the kicker. The car is sitting on old Superstock B-body springs. If that spring has added a touch to the wheelbase, then that last shaft may not help you, and it may be too cramped.