you need to work on why the car is 60 footing poorly at the weight it is, and the ET it runs.
Would need to see what cam, convertor, gears, Heads, suspension etc you are currently using.
My car, for example( nothing special) typically runs 11.40’s in normal summer air around here at 3300 pounds. Goes 1.52-1.55 60 foot, and its maybe 10 to 1 compression smallblock. So lots heavier, less power.
Going to a 2 speed makes convertor selection and other factors more critical, or else the car will definately slow down, assuming its sticking now.Three speed has more room for error in it to enable a better 60 foot unless you have big power you are dealing with. A high 10 sec, light car doesn't have big power, so ironing things out should be pretty straightforward