Weiand water neck???

Yea I ran into the same issue with one of my intakes. From 1951 to 1978 dodge used a 2.4ish inch thermostat and in 1979 they switched to the smaller 2.1" style. Just check with the parts store for one from an older year. Bring yours for reference. I'll add a link to a forum post talking about it all.

http://moparforums.com/forums/f79/thermostat-housing-information-11404/

I can't remember where I got the thermostat housing but Hemitheus is right, it'll have a notch around the housing.