Check your water temp at the bottom radiator hose . . If it's not 140° or so, you're putting the water through the radiator too quick . . Put the restrictor back in . .
SBC Chevy Temps: @ the Thermostat housing 185-190°
@ the Sending unit on the Head (or Plug) 155°
@ the bottom of Radiator or hose 140°
And it don't matter whether it's a filled block, an aluminum one or the one in your Pickup . . it seems to work out the same . . .