Those rear throttle blades can pick up a little carbon and hang. "Blipping" them manually against the slack in the linkage can usually clear that away and allow you to notice when they start fully closing again. Backfiring is a prime candidate that causes this. You need an adjustable canister with a diaphragm that can be adjusted using an Allen wrench. This will give more control over timing advance at vacuum and engine loading. Ported vacuum should be connected to the canister. I would just disconnect and plug the vacuum to the canister for now. 16 initial with a 12 degree distributor gives 28 degrees of timing - good enough for now at 3500 all-in slope, just not as economical for gas mileage with a properly set vacuum advance. I would have just bought a MSD distributor. They come with the slope already set to 4000. You pretty much only swap in an adjustable canister and just work at getting the vacuum advance right, and just use the black advance bushing that comes with it. It's been a long time since I had a Chrysler product. I had a new '69 Road Runner I liked, but the new '70 'Cuda I had was a lemon - something was always broken on it. That 'Cuda turned me away from them, and I never bought another Chrysler product. I wish I still had the Road Runner with it's big wedge engine, but I really hated that Hemi 'Cuda. Both of them would be worth a fortune today, but I sold them for almost nothing back in the day. People didn't care so much for them then. I had my 'Cuda for sale on a car lot for months and it didn't sell. I pulled it when people started stealing the emblems off the body. It took two years to sell it. Both were 4-spd pistol grip shifter cars with only about 20,000 miles on each of them.