If this was done at a shop, I'd take it back ASAP.
Possible things to consider are:
How does it slip exactly? If it's more of a chatter, that can be normal while organic disks break-in. A new flywheel can take a little time to burnish.
Is there any sign that the rear main seal leaks? If so, clean or replace your PCV components. They evacuate crankcase vapors. When they clog up [the breather screen under the grommet] or become restrictive [the spring in the PCV valve breaks or the valve gets varnished/coked], crankcase vapor finds the next least paths of resistance. One such path is the rear main seal.
Otherwise, please provide more info on the exact symptoms, including sensation, if it worsens when hot, cold, at low RPM, high RPM, in certain gears, etc. Again, definitely get the shop involved if it was done professionally.
I don't have experience with the cable you're using, but you probably need a long adjustable cable for the Ford quadrant (correct me if I'm wrong. I needed an adjustable cable for my old SVO quadrant, for instance). I liked the Ford adjustable cable if you need an adjustable version and wonder about your existing cable.
And welcome to the boards.
