You'll know when you take your head off, by how far the valves have pounded themselves into the seats. It takes thousands upon thousands of miles to get to be like that.
If it has mechanical lifters, you might notice it, by more often lash adjustments.
My opinion is that if yur gonna put 12,000 miles a year on it, ok, get hardened seats.
But if just an occasional cruiser , then I wouldn't bother. The car will rust out before it needs a valve job.
But
if the head needs a valve job now, there is no better time to install hard seats.
Plus, I know that you're always on a budget, so, it's cheap enough to do a valve job 6>10 years into the future.
But
you'll know by looking at the installed valves, if it needs to be done, and once the valves are out, it will be obvious.
Oh and BTW, hardened seats will not make it run better of themselves. Just a lot longer at the same good, lol.
But, .100 of the head is a heckuva thing.
If my math is right, that will take an 8/1 225slanty to 9.5Scr. With a stock cam, the pressure is predicted to rise from 140@sealvel, to 175 with a stock cam. I imagine and hope, that you're gonna cam it up to drop the pressure down into pump-gas territory.