Is the mod your making a STC mod for C models or something your working on for the STC mod from the FAA? I have a 1970 C with the typical problems of the rear cylinders heating during the initial climb. I do all the typical things to cool the engine in the climb (increase airspeed to over 120 as soon as possible and step climb to altitude), the mechanic has swap the plugs/probes and checked the engine numerous times. The overheating is not real bad. Typically in the low 400's and mostly the #4 showing up as the problem. The engine is fairly new with less than 200 hours and pretty much firewall forward is new including the doghouse seals.
The aircraft has the lower intake cover (Lasar mod) and 3 bladed prop. I seen some people have had success with the cooling issues but typically only after converting to the 201 style front end completely and strangely some people don't seem to have issue with cooling with just the stock equipment.
So your Mod looks interesting. Do you need to replace the entire front cowling or this just a mod to the plenum box.