69 and down is driver's side exit pump and timing tab on the pass side.
70+ is timing on the driver's side and outlet on the pass side
this coincides with the location of the stock radiator outlet.
so, if you change any one thing, all of it doesn't work well together.
cost wise, the 70+ stuff is more widely available and tends to not command the "premium" of the 67~69 junk (notably power steering, crank and AC pulleys).