Adding weight just behind the drivers and passengers seats works very well for a close friend who races NSS 9.25 index with a 64 Belvedere. Car has gone 9.02/151 in cool weather so slowing it down can be tough at times. He can add up to 130 pounds if needed. The weight doesn’t affect the reaction time and has very little affect on the 60ft.
He keeps a very sharp eye on the weather and knows when to add weight and take it away based on the weather station. He has won over $30,000 both index racing and et racing with the 64. He is a very good example to study. Just search for Doug Wright in NMCA NSS or GLSSA. He has done very well in both sanctioning bodies.
I agree with these guys. Weight is the easiest. Do not use timing. Buy the time you get to where your actually slowing it down without affecting your lights, you will be possibly heating up the exhaust valves as slowing that timing lets the cylinder fire as the valve is starting to open, so you build tremendous heat. Its a good way to hang a valve. Going from 6 quarts of oil to 8 is another avenue. It was worth 3 or 4 hundredths in the 1/4. And 50 weight oil certainly slows them, just make sure it's warmed up before you hammer it... We used restrictor plates under the carbs or behind the throttle body's, but you would need practice with it as knowing how much to use would take a few runs, and it does effect your lights.
Nobody is just going to be able to throw out a # of degrees of timing that's going to kill your et two tenths. I kill my car from one to seven tenths depending on what class / index I'm running. I've been doing this for about 12 years/ seasons. Tried every method under the sun to slow the car consistently and efficiently. What works for me is shifting the car on time and pulling timing. I have hundreds of runs and many years of , car data and weather data to get the timing and shift settings right for every et i need the car to run in almost every possible weather conditions . My system also allows me to change the cars timing and shift set up while strapped in the car up until the very last second before I stage the car. You can't do that with most any other way of slowing the car down that I know of. Most other ways requires the hood to be off/up , or chucking weight in or out of the car. It's not as simple as how much timing retard kills two tenths. Atleast it's not if you want to be consistent and win. 99% of the time I pull into the track, check my weather info , bring up my run records for similar weather on my laptop. Every run I've made has the timing and shift settings attached to the run so I know exactly what settings I need to put in the car. Then just monitor the weather religiously all day and make slight adjustments as needed and the weather dictates. If I'm running a 6.50 index class I usually hit with in 6.49 - 6.51 right off the trailer everytime. Attention to detail , data and lots of passes to dial in the system. Everyone's different and everyone has a different way of doing it. I can tell you what slows my car down 2 tenths but it's going to do you zero good. Especially with weather being a significant factor. Well that was long winded lol. Anyway best info is no matter which way you decide to try and slow your car down you need to go test that way and figure it out and keep records so you can repeat your results if you run index classes again in the future.
depending on what your car weighs It'll take over 200lbs to slow it that far. Probably over 250lbs I'd pull the carbs back to get it close. Might even get there by disconnecting the secondary's. Then adjust with less weight placed in the center of the car. With this method you can get it with in .00x of the index. It wont mess with reaction time enough to worry about.
Thanks for the advise everyone. I figured I'd have to add about 200lbs and didnt think I could do it safely. I ended up pulling timing. 10 degrees. I had it just about dialed in, in time trials and then had a slick blow out on return road so I was done for the night.