MSD distributor tuning

God morning all!
My 87 Ramcharger has a MSD ready to run distributor in it. It's a stock 318 other than dual exhaust and 4bbl.
I just cant get more than 30* Mechanical advance out of it. Initial is at 15* and with vacuum it go to 50*. I changed the stop bushing from 21 to 25*. The springs I run is 1 heavy and 1 light silver springs. I feel the heavy silver is limiting my advance. If I take out the heavy for a lighter, it will ping. (Heavy truck)
I was thinking about trying to opening the C-C distance of a heavy spring, a few thousands to hopefully give me some more. Any other suggestions besides doing this? I also toy with the idea of using a factory Mopar spring with the elongated end (smog limit spring?)
This truck needs more timing and responded well to the 25* bushing. I will probably go with the 28* after I get more Mechanical out of it.
Your thoughts?