Just to check for head-gasket problems or cracks if you haven't already done this, see if you can get somebody to stick an exhaust- gas analyzer probe into the radiator fill to check for exhaust gasses which may be there even in the absence of bubbles. It's a bit of a long shot but that's one more thing you could eliminate.

Unless the block is totally filled up with stop leak, you should be getting *some* flow. The bone-yard 261 I bought had a couple of inches of black, hardened stop-leak in the bottom of the block and I had to chip it out with a long screwdriver.

BTW there is a soft plug in the back of the block behind the bell housing that you can't get at even with the bell housing removed. You have to pull the engine.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.