All good points. And you left out checking the plugs and pistons and fuel flow for signs of lean operation.
Our plugs were always nearly white, no scale on the pistons and fuel flow was almost 2 gal/hr below what is specified.
Before we would cruise between 380F-405F and without powering back have no idea what the temps would reach but they would go sailing past 450F towards meltdown.
Don't know how much of a difference opening the cowl made because we could never leave the in all the way, but like others, the CHT drop by increasing the opening 5/8" was over 30F and probably at least 40F-50F
But we were still lean like nearly every other carbed D1A O-320 9A.
After chasing all of those other things for a year first then opening up the cowl and jet, now we cruise below 380F all cylinders and almost alway climbing below or near 400F.
Somewhere on here there is a link to an article from an EAA member that did an excellent job of documenting all of the steps he took chasing this problem which was cured by opening the outlet and drilling the carb just as it fixed ours.
If you are committed to painting first, and don't mind the look of louvers then I don't see any down side to installing them during the build. If you see temperatures that are lower than you want you can always block them off.
Except that louvers may not be as effective as opening the outlet and installing a fairing.
I have a 6A with an O-320 and suffered from high CHTs until I added louvres. I believe the outlet hole is too small, so there is not sufficient airflow through the cowling. The louvres dropped my CHTs by at least 40F ... My view is that, yes, louvres are required, especially for a nose dragger.
Pete
Yes in many cases the flow is too little, but I think that there a number of fixes that show that open the outlet is at least as effective. I think after much research most folks that install louvers don't place them in the proper location (e.g see Pipers and Bonanzas)
Does anyone have a carbed O-320 9 that runs too cool? haven't see one.
There are a number of NACA studies that are available on the net that address cooling in more detail than most folks would care to try and understand.
You can chase it as long as you want ... or just fix it...
We stopped chasing it after about a year of trying all of those other things... As soon as the engine warranty we carefully increased the jet...