There is a very simple way to bleed the air out of the straight sixes- remove the tempurature guage sensor from the top of the head, start the engine fill it up with water. Eventually all the air will bleed out of the block and cooling system, then you replace the temperature sensor.

Lots of the newer thermostats come with the small hole metioned in the other post. If you are shopping for a new one, you might keep an eye out for one with the hole.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi