I also have an older C8 tube that was once attached to a Nexstar single-arm mount. My problem was the same as yours, none of the OTA holes line up with that aftermarket dovetail. The newer C8 tubes have narrowly spaced holes in the rear cell to match the narrow dovetail.
I didn't want to drill and tap my OTA. Too much risk of damage.
Fortunately, I was able to drill and tap the existing Nexstar single-arm mounting bar to match the dovetail eg I used the original mounting bar as an adapter.. This gave me a useable but not perfect dovetail attachment. Maybe it's possible to get a 6mm thick aluminium plate from an engineering supplier and drill it to match your application. May be better than drilling the OTA.
I finished up getting a good strong C8 dovetail from Ken Dauzat in the USA
http://www.kendauzat.net/other.htm
I haven't attached it yet becaude my existing solution is adequate and I'm thinking about a move to wider Losmandy-style bars.
Cheers,
Rob