The 401 was not used in pickups, usually found in 5 ton trucks. You might want to ask in the Heavy Haulers sub forum on this site. I believe the 401 used the window in distributor cap with allen wrench access to point adjustments. The 305 that was used in pickups has the non window distributor cap. I recommend a shop manual like this one ( heavier duty trucks) 5500 and up.
http://www.ebay.com/itm/1960-1962-GM...ccd04e&vxp=mtr