tires spiing only in 1st is a huge problem... For every tenth you save at the 60 you get 2 down track. Your MPH is off too. Did you up the center jetting to 64? I've seen guys run 11's w/ 5/16 line, mech pump and a 6 pack so I'd check things like timing and carbs 1st. I'd disconect the vac advance and set your timing at 38* total at 2400rpm. Then check your vacuum readings, adjust it so you pull the best, set the idle, re-check your total and take it for a romp. I think that back in the day low 14's high 13's were what those cars were pulling on bais plys thru the pipes.
What else did you do? headers? bigger exhaust? mufflers? what are you shifting at? all that makes a difference.