[jgpnashville]

James and Jerr - thanks for the ideas. I've been thinking it's running rich, but that makes sense why it would be OK rich when cold, but misfire when it warms up. I'll definitely check that, starting with pulling the plugs.
Meanwhile, I caused a leak in the radiator when draining it today to flush and replace the hoses. The drain plug was seized and turning it bent the copper rad and the female thread for the plug now has a leak around the fitting. I pulled the radiator out and will take it to a shop to be repaired (hopefully) and I'll drop it back in. Got a little more than 6 gallons of nice green coolant out, less than the 8 gallons I was expecting. So, it's not in a running state right now.

A question on the water manifold - if I remove it to replace the short hose between the water pump and the manifold, where can I find the gaskets to seal the water manifold against the head? Anyone know a part # for those gaskets?