572 cylinder head overheating

[Denvervet]

I've got an overheating issue that an IR gun has narrowed down to one particular cylinder. I'm about to tear the top of the engine down for the third time and only have 100 miles on it. Cylinder ...#6... runs 30-40 degrees then comparable cylinder on opposite head
Setup
Crate 572 gm
Manifold single plane modded for fuel injection
Ron Davis radiator
Vintage air frontrunner with stewarts water pump

What I've done to diagnose:
Had to rebuild engine at first startup due to a metallic object being in #6 and had to replace entire head and piston
Ran into head heating problem and tore entire engine apart and had block pressure tested and deck cleaned up to insure true surface. Replaced all bearings,crank polished,etc
Same problem
Put bypass hose as well as tried without thermostat as per Stewart's advice....no change
Exhaust gas in antifreeze checked twice...negative
Plug showing real lean so bumpd fueling up 50% via efi...no change
Head in affected area gets up to surface temp adjacent to header of 270+
Intake water temp creeps up to 215-224 without pushing much and temps only drop 2-4 degrees on highway when 75 outside. Placed rubber seal between Rad support and hood
Sealed Rad to support
Changed out fuel injector to rule out lean cylinder due to faulty injector...nothing
driver's head commonly only runs at 205 when good and warmed up prior to entire system overheating
Thermostat stewarts 180 with bypass holes
Jacked front of car up to the sky and ran to be sure all air out of system as well as pulled head temp sensor while filling
Timing initial 17 degrees....advancing of no benefit
Used factory head gaskets...and only one way to put them on.
Tried the propane trick to look for vacuum leaks..nadda



At this point concluding something obstructing head coolant flow....head or block casting
I'm hoping someone has another idea of what to check before I start the tear down again!
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