You have 16 base, 21 mechanical totaling 37. If you are using the stock MSD vacuum can, that will give you too much advance at light throttle - around 52°. You can either:
1. Run it without vacuum advance. Lots of people do. It is designed for fuel economy while cruising. Not a big need in a cobra. No need to switch out to your old distributor. You can leave the can disconnected and it won't have any effect. Just make sure to plug the carb side so you don't have a vacuum leak.
2. Change out the advance limiter in the distributor. If you use the black one, you will only get 18° of mechanical advance. Set your total to 34, still giving you a base of 16. Those few degrees *may* keep it from over advancing. (Doubt this will work with the stock MSD vacuum can. These number are for a SBF - YMMV)
3. Replace the vacuum can on the distributor with an aftermarket adjustable piece. I use the Accell 31034 unit (
Ckicky - get the GM part, not the ford part) and can limit the vacuum advance to around 7° instead of the 15° the MSD one provides.
BTW, I recently switched to a CB Performance black box to control my ignition and can set any timing at any RPM/Vacuum.
Forget the inline smog vacuum reducer. Waste of time, money, and effort for the limited benefit it will provide.