The 74ls14 is only needed for the repeater jack. The article I referenced said that with the 6n138 sometimes the midi commands didn’t get through because of slow edges from the 6n138 (that is what the resistor on pin 7 is about, it speeds up the rising edge, but may need adjustment). The 6n137 is 10 times faster than the 6n138 with an active output ((the 138 needs a pull up resistor which is why the edge is slow). If it was me I’d use the 6n137 as it should be more reliable, and it doesn’t need the 74ls14 to drive the output.
Peter