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We used to rebuild a lot of these yrs ago and for the life of me can't remember if we used early model timing kits on these . Rule of thumb here. Advanced timing increases low end power, retarded is for higher rpm. Yes, it was done for emission purposes as retarded timing produced more heat which burns off excess hydrocarbons. Higher combustion temps unfortunately produced higher oxides of nitrogen....hence the need for an egr valve . Without the egr valves 351m and 400 "pinged" on light acceleration. Bottom line if you are going to do a chain consider a mild cam to go with it. That's the best way I had to curb the ping. You don't want to advance the cam timing and then "de-tune " it with ignition timing to kill the ping. Your vacuum advance is adjustable which helps if you just do the timing components. Your altitude weighs in on the ping thing as well. Good luck
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