OK...i'm officially stumped. This is the same engine that i showed a pic of the burned oil on the rocker arm a few weeks back. Here's the setup: 68 400. I pulled the heads over the winter & had them tanked/cleaned/hardened valve seats installed/pushrod holes elongated for 1.65 rockers. I also installed a new Performer intake as well, new Comp Cam (268H) with lifters, new water pump, & Comp Cam roller tip rockers. I also have a new full MSD ignition. I start the engine, temp goes to 160, thermostat opened cooled the engine. If it idled longer, or drove for a short period, temps climbed over 220 & up & up. I then intalled a new original 4 core big block radiator, a milodon 180 high flow thermostat, & a 16" electric fan to the front that pushes about 2100 cfm. I double checked the timing & it's right on. I started it with the changes & the exact same thing, completely overheats. I'm stumped. My last & only guess is that a water passage is plugged, possibly the head gasket or intake gasket? Any ideas???