Finetuning questions - do I need a high-stall torque converter ?

3.55 gears and some decent tires should get you into the low to mid 13 at least . Seems tire spin is hurting your 60 ft times . And you should try shifting @5800 ,but a bigger carb would be needed 650 at least . that 500 cfm is ut of steam early @5200 rpm max hp should be around 5600 to 5800 rpm .

with no speed limits get an overdrive if you change the gears , always fun to find the top end of a car . and a trans cooler if you get a high stall converter