I read thru this and I have a couple of things for you to try. If you have different bushings for the distributor put in the biggest one, this way the distributor will not advance as far and you will be able to have more initial advance. Keep your total down to at least 28 or 26 if you have to. You should be able to get to 30 but maybe not, the gas could be crap out there how much ethanol is in it? If 20 % you may need some more jet in. Try using spark plug NGK YR5 I use this one for the nitrous and this is basically the colder plug for the 1970 350 engine. This may help you creep back up on timing. I had seen in one posts that you were down 2 qts on the oil. You could be burning some oil and If this is so, this will reduce the octane in your gas and you will ping. When you put in your heavy springs in your distributor it did not ping because you had to hit a higher rpm before it would ping, this may have been as high as 4500 rpm with the heavy springs. You should be able to run the medium springs and get it all in at 2500 rpm. Also with the heavy springs in you bog off the line unless you are using the vac advance then it will not bog. If all else fails run 5 gal of race gas with your 15 gal of 93 and this will help out the ping problem This may help you get back up to 30 to 34 on the timing and it may just get better gas mileage as compared to running 26 on the timing.