It's not a big deal with a valve spring compressor (rent it at autozone), compressed air, and a stick magnet. drop the fan and shroud and rotate the crank to put the cam on the flat spot for each cyl.
At that miles, honestly you might want to just swap heads- they're tired and most likely will need valve stem guides and could be cracked- those are lightweight castings, a major overheat will cause failure.. You can get replacement heads all day long for $200 /
Pr anywhere. Had we had this conversation 6mos ago I would've had some heads for you- fresh, sold 'em tho.
If you do the replacement heads, make sure you stay away from Vortec castings or aftermarket, no EGR provision, which is what you need to stay smog legal.
One last thing- when you reassemble the rockers and need to adjust prelash, BE VERY CONSERVATIVE with the preload. It's easy to overdo it. slowly tighten the rocker arm while you wiggle the pushrod back and forth (cam on heel, not on lobe obviously) - when the pushrod and rocker literally have zero slop (the moment they touch), go maybe 3/8 a turn more, and that's it. It's pretty sensitive. If you overdo it, you'll hang valves when the engine warms up.