Hot at highway speeds
A little info found digging around. Some of the reasons I bought that Robertshaw in the ebay link above.
" The 330 series was their hi-performance line (330-160 & 330-180) that fit many GM and Ford engines. The 330 series had a 1.5" hole when it was open versus about a 3/4" hole for a standard stat. "
It is a balanced flow stat and fails open, also.
They are not easy to find. Summit and FlowKooler are both out of stock.
TX Dart and I were discussing your post and both think the thermostat being pushed closed could be part or all of my issue. Thanks for the insight. Will post back after install.
Edit: Looking at the Milodon and Mr Gasket, I don't see much difference. Looks like the same design. Probably the only differences are where it's made (US,Mexico,or China) and whether or not it has bypass holes. I have read some Mr Gaskets are repacked Robertshaws.