What PSI should the cylinder be for valve stem seal replacement?

[Skyline]

Also, most junkyards offer some sort of limited guarantee on their engines. You can certainly do a leakdown test before installing the motor once you have it. You could possibly do a compression test if you could rig the starter. If the junkyard still has it in a car, all the better; you can hear it run. I doubt most yards would let you do any tests on the motor, but you could ask. If I had access to an engine in a car, I'd be happy with a good compression test, and one of those coolant tests that can determine if combustion gasses are in the coolant.

I wouldn't swap heads if you get a motor with 60k miles. You could drive it for several years before having to do the valve seals, and you may have parted ways with the car before that point.