A '74 F head run for approximately 60k miles undergoes a 3-angle valve job and is put back into service without hardened seats for approximately another 10k miles. The head is now off the block and the valves (just by looking at them from their top since they are still installed) seem to be sitting pretty high and not sunken down into the head material. I'm thinking of just cleaning up the head, replacing the stem seals, and putting it back into service. Would lead substitute help to 'self harden' the recently ground valve seats once this thing is running again?

Deluxe

I had been using a lead substitute because I wasn't sure what valve seats I have in my 67. I also use it in the 62 which has a factory rebuild in it from 1993 so I asssume those are hardened but I used it there too, just in case. Then, a couple of years ago, I switched to using Lucas Injector Cleaner which promises to lube the valves while keeping the valves and carbs clean.
www.youtube.com/watch?v=VlitplNaxjo

Recession happens when you run the valves at high heat for prolonged amounts of time. 99% of the time you don't need any lead substitute, the only time you really need it is when you are flogging the engine for extended periods such as climbing mountains.

If you check your tappets regularly, you'll be able to know yourself what's required.

My recent experience is more in line with this post by b20paul. I always have lotsa stuff in my 145 wagon. had to adjust my valves 2 times in 3K miles, just started adding lead substitute every other tank full, it's running better than ever now!

If the head you are using is indeed a 74, and was originally installed on a US model, it should already have hardened seats as standard equipment, either on the exhausts only, or on all valves (I've seen them both ways) You should have nothing to worry about.

Ignore the fact that the seats are not hardened. Just give it a good valve grind and be done with it. As you have noted, the valves sit up high, meaning the seat widths have not increased. All of the exhaust valve seat recession I have encountered over the years is the result of weak valve springs and worn lifters. Both allow the valve to bounce on the seat on closing, and that takes out the seat.
I did lots of valve grinds back in the day for notch burning of exhaust valves on engines running on leaded fuels. Since then that kind of work has dropped off, because unleaded fuel prevents the deposits that cause that type of burning. I've had seat installed on engines that experienced recession of course, but it's not the result of unleaded fuel.