I have a '70 Six-pack manifold, stock thermostat housing. It has the large thermostat and housing, got the thermostat from Flowkooler (Robert/Shaw). There is no room for the o-ring housing, the thermostat and bolt holes are right next to each other. So there must be manifolds out there that are using a smaller thermostat and housing that would allow a machined groove and o-ring.
