I have no experience with the Car Clinic 3.4 Essex. But have built a 3.6L Essex many many years ago. Off-set ground crank to 78mm and massive overbore. Pretty much used Chevy parts for that conversion. Chev rods, pistons, valves, valve springs, screw in rocker studs and roller rockers.
Another option is to source a South African 3.4l Essex. They used a few Cologne parts in them.