Dave,

I guess what I am referring to is the "bypass orifice", a brass disc with a central hole, which on the Cleveland sits in the well below the thermostat on top of the front of the block. It has "Top" stamped on it.
I have just read (in the Hot Rod book covering the 351C & Boss 351 engines) that this part should never be removed, even if running no thermostat, as most of the coolant will then flow directly into the water pump rather than to the rad, causing overheating.
Sounds like I've answered my own question - unless anyone out there knows different...
Chris