Cut it out and probe again on the stubs ( the board will still not work, but you will see if the TTL is doing anything on the board. Measure both from Vcc and GND, to detect weak pull ups and pull downs with the 10M meter impedance, so you can see floating lines.
Put in a socket for the replacement if you can, though with the one board I worked on the official IC replacement measure ( aside from changing the entire vapour phase reflowed board at a price of $FARK) was to cut the legs off at the body ( or destroy the CERDIP package and remove the ceramic on those, we were FUBAR with the side brazed ceramics, but luckily they were not our trouble children) then clean the inner remains with a needle file and shape the new IC to fit on those stubs, often by gently filing the plastic package down slightly to narrow it. If you say just desolder it then you try to desolder a DIP package where they essentially used both sides of the board filled edge to edge with TTL packages and used solder paste in the holes as soldering. Done to get the board small enough to fit the case. No LSI in 1970, only SSI and MSI, and custon was not common on Mil spec at the time, they did not want to make a custom LSI for a sub 200 production run. Only the USA where the buyer was the government, and they ordered 10 000 planes as an order, was it viable.