The MC789 is a hex inverter, a 7404 or 7406 could be used but the pin-out is different -- The only thing it has in common with a TTL inverter is the function it performs. RTL logic, power supply, and switching performance are NOT the same as TTL. I have the RTL section of the 1969 Motorola microelectronics catalog scanned I'll see about getting it on line later today.