I’ve been slowing my cars down for various bracket racing reasons for almost 30 years. Both to fit into certain classes, and at times to keep it under the NHRA rollbar limits, first 12.00, now 11.50. Some of the classes I run don’t allow gadgets like electric or air throttle stops, timed stops, etc., so I & some of my friends have used different mechanical ways of limiting full throttle, among other methods. I’ve also tried various carbs. I’ve learned some things that have worked well for me, and some things that didn’t make much difference. One thing I’ve found is it takes a pretty significant change in limiting the throttle sometimes just to make a small ET change. If you think about the throttle blades at full throttle standing straight up, you can get an idea how a few degrees slightly off full doesn’t have a big effect, but as the blades get closer to closed the effect becomes bigger quicker, so my point is just that it’s not a linear change, more of a curve. The same also can happen with the arc the carb throttle arm goes through. I remember reading your earlier posts, but I don’t remember all the details, but I think your car is running a pretty similar ET to mine, and currently I’m running a MR 2-4V intake with the stock BJ/BK Holleys. Going from 11.30’s to 11.50 won’t be too bad, but it does take a significant change. You didn’t mention if you were using the stock Mustang throttle linkage rod, but if you are, one thing I have done in the past to make a small 1-2 tenths change like that, was adjust the threaded throttle rod that connects to the accelerator rod at the firewall so that when your accelerator pedal is on the floor your carb linkage isn’t at full throttle. I use a small hairpin type clip on the pin, instead of a cotter pin, to make it easy to remove & adjust. Leave the linkage at the carbs alone. That will give you a measure of adjustment to fine tune it to a safe ET above 11.50. Here’s a picture showing the part I’m taking about adjusting.
As a more permanent solution, and to give me the ability to close the throttle more so I could slow down to 12.00, I fabricated this - I took an aluminum plate I already had on hand ( from a Moroso throttle cable plate ), and drilled & threaded a hole for a bolt I can adjust, which the stock Ford 2-4V jackshaft contacts to keep from going full throttle. I recently ran this, and after a half dozen passes I got to 12.00, but it requires closing to less than half throttle to accomplish that. I keep a cheap caliper or small ruler with to measure the height I have the bolt adjusted to, and log it in my logbook.
Personally, if I were you, and guessing you will be limited on the amount of time trials you can make, I would not start with your plan of making a full throttle run, and then slowing down, but I’d start with a significant amount of linkage adjustment first, to try and safely be under a 11.50 ET, and adjust from there. That will also keep the track from scolding you, and gaining unwanted attention immediately. My guess is your first attempt may still be too fast, but if not, it’s easy to adjust a little quicker if you want. What I would do is put a small piece of tape on the threaded rod, to mark where originally you had it, to make it easy to go back to your initial full throttle setting. You can also use the tape as a spot to measure from, and log the settings.
You may also want to consider some legal ballast. You can take a heavy truck spare tire, the biggest you can fit in the trunk, and secure it with the spare hold down. I’ve done it in the past when I’ve been away at a distant track, and just need to take off a half tenth, by using my tow vehicle or trailer spare. I carry my old Bronco heavy duty spare tire J-hook with to use as a hold down. It’s not NHRA legal to have loose ballast in the trunk, and it’s a dangerous idea anyway, if you’re in a crash, so be sure & secure it. As a more permanent solution you can add weight bars, or a weight box to the car, there are guidelines in the NHRA rule book. That can help you adjust the ET .1 to .3 fairly easily and consistently.
I like Larry’s secondary limiting ideas too.