Crack in the manifold is key here.
Also, when you replaced the t-stat did you burp the system to relieve it of any air pockets (which usually occur in the vicinity of the t-stat housing and inhibit cooling system performance)? If not, run the engine up to operating temp with the coolant reservoir cap off, and when the t-stat opens up grab the upper hose and squeeze it a few times to displace any air in the system.
If you did not do that procedure, that is also a likely cause as to why there's a crack in the intake manifold. The good news here is, when you go to replace it you can use a PI manifold and get a nice little bump in performance out of the swap.
2001 Pussy Slayer Red GT convertible; all of the usual bolt-ons.
1967 Notch; 200 I-6, C4 auto