My 1967 383 2bbl may have had a set of 915 heads as original, not the old 516 383 heads like it should have come with - engine looked to be untouched - and when I gave the machine shop the new Mopar 1.74 valves they said they were too big for the seat. I could only guess that the line ran low/out of 516 heads that day and installed the 915's, with the smaller 1.60 exhaust. A 'throat cut' was all that was needed to have the 1.74's installed. Either that or they were installed sometime later without the original owner's knowledge - either way, a nice step up from the 516's and got the motor and trans for $100!