If you want to slow a car down easily with out killing the tune up, change the rear gears. going from 4.10 to 3.90 or getting a second set of tires with 6" more roll out if you can fit them will probably get you there if you need to gain a few tenths. adding some weight in the rear bumper area will also slow a car down too.
Gears are cheaper than a carb, changing the carb will likely not change your et as much as you hope.
Bruce