could be the vacuume advance or the fuel pump spring is loose (not enough tension on the cam eccentric). this could cause it to slap on the eccentric. my engine is similar the lifters seem to be pumped but then one or two chatters slightly then disappear. i have comp plus cam set with 3 springs. i cant figure it out. what is your compression? could be too high.

68 400 h.o 4 spd to go!!