According to the 1966 Master Parts Catalog, there is only 1 driveshaft:
part #2533201, 61 5/8" long, 3 1/4" diameter. I should say that this is
listed as an assembly including shafts with center joint, rear yoke and
flange only. The MPC lists 2 different front yokes: a 9" length and a 7
1/8" length. No qualifier given, and since the 9" is also shared with
the B and C bodies this may be a misprint. Also, as someone mentioned,
the convertible does have a different center bearing INSULATOR part #.
NOTE: the 1965 MPC shows exactly the same number for the shaft assembly
(but you'd have to change the front yoke) to fit a 66.