Mustang cooling issue continued

Back a few months the forum was very helpful in assisting me with a rebuild of my original Sherwood water pump (thanks again to those that offered advice).

Here’s a quick recap of what led up to the rebuild:

I noticed my RWP didn’t have enough suck to pull coolant from a bucket when the short inlet side hose was removed and put directly into a bucket during winterizing. This made some sense as the boat has never liked going slow and temps would start to climb while just cruising around 1000 revs. When I went faster, temps went back down. Now that the water pumps been rebuilt, emptying a bucket of water takes maybe 15-20 seconds, so things are certainly better. Despite this, I’m still having some cooling issues and want to keep digging until it’s resolved.

I have a couple new observations that may help.
I was recently pulling a novice skier on 2 at 25 mph and the boat ran great. Stable temps and he went for a long time. Then it was my turn to slalom at tournament speed (36). After a couple passes I could feel the boat slowing and this was coupled with a little bit of black smoke and some visible white steam out the exhausts from some mild overheating. I’ve seen this several times before on my boat and have been chasing a solution for years. When I asked the driver how the temp looks he said it was just slightly higher than normal (120-140 is normal, showing maybe 160). We made it back to the dock just fine with temps running slightly above normal (again, 160ish). I pulled the cover up and could see water flowing through my clear Flush Pro. I couldn’t see anything else that looked out of place. Then I happened to do a quick check of the exhaust manifold temps and found something interesting. The port side manifold was just a little too hot to keep my hand on permanently. The starboard side however was much hotter, too hot to really touch at all. This got me thinking about the water flow path and a brief mention of T-stats in the water pump rebuild post. As you can see from the pic, water comes up, T’s, goes L & R and the flow towards starboard goes through the T-stat. This makes me think I may have a thermostat problem and this is resulting on some overheating on one cylinder bank only.

Once again I’ll ask the experts here what thoughts they have on this issue?

Got the 4th coming up and would really Big Jim to be as reliable as possible.

As always, thanks Tim. Can't see the pic of your 66 stat, but I'll take one of my original rusty stat tonight for comparisons sake. What does drilling the weep hole accomplish (better flow)?
Ordered what I thought was the correct stat and gasket from SKIDIM and of course, it's not the right one and needs to go back (the gasket at least). At least I know this isn't related to my overheating issue (pretty sure that's what your saying). Maybe I didn't look close enough at one of the cooling hoses pre RWP. Pulled'em off at each end, but didn't remove both ends and hold'em up to the light to make sure the entire hose was clear and clean. If the issue isn't the hose from bottom of boat's brass intake to the strainer, strainer to tranny bulb, tranny cooling hoses or tranny bulb to RWP then I'm pretty much back at square 1.