just swapped motor/cooling?

[429mustang]

I'm not gonna lie, this is a question for a chevy motor. I just swapped a 355 into my 85 astrovan: (perf. rpm cam/intake/600 carb/ported-polished-milled heads/headers/gear drive/roller rockers/accel hei/weind water pump,alum. 2 row radiator). I got it running pretty good, although I'm afraid to drive it around too much yet, it seems like it's running too hot. I'm running a fairly large aluminum radiator with a good pump and a stant 180 thermostat, but the temp gauge shows around 210-220 after 5-10 minutes and still seems to be climbing, and i've alreeady got the air out of the system. With the cap off, it looks like it takes a while for coolant to start flowing good. Anyone have any ideas? I'm thinking it's possible I got a bad thermostat, but I wanted to see what other people think first.