The first thing you need to do on any old 6pk carb that has tight screws is get the old cork seal out.It gets rock hard from age and first thing that happens is off comes one side of the screw head.Probably 75% of the carbs I get for restoration have at least two broken screws.I made a drill pilot that enables me to drill the screw on center,have a four sided pin punch,tap into hole,heat base around screw and 99% have came out.If you drill without a pilot and get off center you can plan on scrapping the base.
You can get the screws from Chicago Corvette Supply for less $$ than from Holley.Part # M1513.
http://chicagocorvette.net/If you only need one,PM me and I'll put one in the mail.