Violent SL3 HPRs are phenomenally consistent.
Only problem is they're quite overpriced.
A suggestion on the side would be a Bob Long 360 Torpedo. Relatively as, if not more, consistent than the SL3 and much cheaper and lighter weight. Also simpler for maintenance.
Nice cocker chopping already, although I despise chopped cockers in general unless the bodies are essentially blanks...
Sidenote: You can use any flat-topped frame in which the screws will line up with that of the cocker's. If the screw holes are off, you can always tap new ones in the body anyways. Just be precise about it