66 GMC?? Probably a 250 not a 235. Offy is the preferred intake. Headers help. Clifford makes some or Langdon's. Carb wise anything up to 500 cfm works. Some use holley 390 cfm ones.
I run a holley 350 2 barrel on an Offy with Langdon's headers on mine. Starts instantly and never misses a beat.
