I have an older Carrier natural gas furnace (1970s). Pilot doesn't stay lit. New thermocouple didn't help. Repair man says I need a new gas valve, $700 plus install, suggests replace entire furnace. Can I replace valve with universal valve and save $$?

No... You have to use the correct materieals rated for exposure to gas. Not only would it be dangerous... it could void your homeowners insurance. Work on gas lines most places require people certified to work with it.

As long as the working pressure is the same and your valve is listed as an exchange it should work. Check prior posts that address this issue.

Yes, almost all gas valves now are universal