I had a similar issue with my 261 at first firing, My issue was the fan belt was not tight enough to spin the waterpump fast enough at idle.
It is worth checking out...
SWEET Sergeant At Arms: Old Dominion Stovebolt Society BUNS?!?!?!Where we're going, we don't need no buns..... 1950 GMC 450 1951 Chevy 1/2-Ton The GreenMachine In the Stovebolt Gallery