There was an FE style tank on my six cylinder 62 Ford wagon. I had the impression it was there to accomodate the low hood. Especially where the hood started to roll back from near vertical to horizontal and was very close to the radiator top tank.
Many of the newer GM's have a bleed screw in the thermostat housing. Should be easy to drill and thread a hole and add a plug.
Bingo! It was that and the lower hood profile Ford was going with at the time. I've seen a few guys remove them in the past (for no good reason) and wonder why the headers glowed red and it overheated like a bastard. Bob