I put a Holley 2bbl on my son's 66 GMC (305 V6), which dramatically improved how it idled and ran. Mr. Gasket sells a $15 adaptor plate that I believe changes small 4 bolt Rochester to Holley 4 bolt. Carbs are about $280 new from Summit or Jeggs, but then they are rebuildable and you can find information on tuning them. See http://www.6066gmcguy.org/holley.htm don't know if it will work with your application or not, but it sure was a good way to get old GMC carb out of the picture.

Last edited by R-Bo; 07/16/2012 4:30 PM.
R-Bo

1959 Apache 1/2 ton Big Window, Short bed Fleetside (under reconstruction)
1966 GMC 305V6 in the family