B20-30 valve stems are actually smaller diameter than on the B18, but that doesn't affect the setting.

Do what you think is best, but loose valve adjustments are one of my pet peeves... what many owners think is the trademark sound of a pushrod Volvo motor is to me a machine crying for help.

No need to look at an old cam to know which valve to adjust -- here's the most accurate way:

Turn the motor in the direction it runs only (as if you were tightening the bolt on the crank pulley). When an exhaust valve just starts to go down, adjust the intake valve on the same cylinder. Look at the intake and exhaust manifolds and you can't get lost about which is the intake and which is the exhaust.

Do all the intake valves first. Look at the distributor and plug wires to see which exhaust valve is going to go down next, following CCW around the cap from whichever one you've just adjusted.

After all six intakes are set, keep turning look for an intake valve to just start coming back up from its full down position. Adjust the exhaust valve on that cylinder. Do all the exhaust valves that way.

BTW, there's no need to reef on the adjuster nuts -- just snug 'em down firmly.