Best way
I have had this issue for 2 seasons.
Slowing a 5.90 car down to 6.40+ to avoid having to get a license again and the associated cost.
I went thru 10 ideas.
Finally used a homemade restrictor plate under the carb at a RJ members suggestion. Slowed to 6.28 with a 4 hole 1 3/4" plate for a 2" dominator carb. It works consistanly. Rather than making a few more to get slower and having to pull the carb each change, I bought an E Z Plate system. It has a 1" spacer with a cut out in the side to slide in 1 of 20 supplied plates. A simple 1 bolt cover comes off and a change can be made in 30 seconds. I haven't tried it yet, but a lot of Super class cars use them. It is very high quality looking, but is a little salty at $350. I think this will be the answer.
No changes to your tune up are required.