Interesting about the Sonnax R servo, I have not had that problem,All very good info.. I will add if you were to use that Sonnax R servo , every one I have ever used has been too tight in the case and requires machining of the larger ring groove
I about to build a coupe C6 and going to try the Superior servo
For a cruiser a D servo is a good choice
but have had the teflon seal and O-ring on the servo stem be too tight in the bore and cause it to bind.
The problem is usually the O-ring.
I think Sonnax corrected that and now puts a slightly smaller O-ring in the kit.
The Superior servo is a good one too.
Superior K016 | Superior Transmission Parts - The problem solvers of the transmission industry