You mentioned a one or two core rad. That would only be the case if you have an aftermarket rad. If its original on a standard 68 350 two barrel, then it's prabably a 3 core.

I bet if you changed to a 4 core or an aluminum with a 1 inch core you would drop it more.

190 is not too bad though. Your thermostate won't even let the water fully flow until 180, so 10 more degrees means your probably close now, you just need to dial it in a bit more. Maybe the radiator change will do it.