The fitment is interchangeable between the '65 and '66, but the cross section profile is different between the two. The '65's have a wider more concave contour while the '66 has sort af a raised rib profile (hard to explain). '65's are hard to come by and not repoped as of yet (as far as I know).
'63 Impala SS 409/425 owned since '71
'64 Impala SS 327/300 factory 4sp p/s p/b air
'66 Impala 327/275 glide 2dr/hdtp
'66 Belair Wagon 283 700R4
'62 Impala 350/350 dual quad th350 2dr/hdtp
'00 Camaro SS 6sp hurst SLP