Resurrecting this thread to add some more info.
13958 is definitely not the right size for the Meziere housing. It's a 52mm thermostat, and the kit is supplied with a 54mm one. The 13958 when placed in the housing will cover the hole, but it's got enough slop side-to-side that it can open up a gap if it doesn't stay in that location.
13958 on left, supplied thermostat on right:
(Yes, there's an empty hole with no jiggle pin in the supplied thermostat. I'm pretty sure it had one when I put it in, but it's clearly not there any more. This may explain why it behaved more like a 160F thermostat than the 180F it claims to be.)
Meziere says it's a "traditional Chevrolet thermostat", which appears to be a Stant 13358/13359. At least, that's what you get looking up a 1967 Camaro with a 350, and it measures out as the right size. The Stant SuperStat (better flow) in that size is a 45358/45359.
I couldn't find the part # for the seal that goes on this thermostat. It's not the 25270 mentioned in RavynX's thread because that's for the 13958, which is 2mm smaller. Digging around at the local parts store, the best they could find was this:
Which appears to be intended for various Toyotas. It's thicker than the M-tuned one, and has a slightly smaller diameter (but still larger than the 13958 with seal).
It fits nicely in the housing:
Incidentally, I had also ordered a new Meziere housing as well, in case they'd changed in the 5 years since I bought my M-tuned kit. They have a slightly different external appearance (different radius on the corners), but internally they're the same size.