Time for diagnostics.
Check timing, timing mark vs. physical TDC on #1 piston.
Check for erratic timing esp abofe 1500 rpms.
Check vacuum reading, should be 21" MSL, adjust for your height above MSL.
Check vacuum needle at idle, should be steady or all but steady.
Pull spark plugs and "read " them for what is going on in combustion chamber.
Check timing chain for slack. Over 7 degrees, replace chain and both gears.
Check fuel pressure at carb with engine running.
Check for restricted exhaust
Did you add a dedicated ground to the black wire (3 wire harness) from the distributor?
Let us know what you find
418 miles on 19 gallons of gas. Can't beat the BBD !