how bad does it smoke??
Nearly every chevy motor I've owened has the "chevy puff" when you first start them after sitting awhile,especially the first start of the day after sitting overnight..my 79 C10's 86 305 puts out a good belch of blue oil smoke upon first starting it,but it hasnt burned a quart of oil in nearly 1000 miles yet,so I'm not going to change the valve stem seals--
-as the last post said,usually the guides are loose too,and replacing the seals,with either the OEM ones,or the "Umbrella" type seals might not help that much--and doing them in the truck isnt that easy a job,you need compressed air to hold the valves shut while you take the valve springs off,(and pray one doesnt drop in the cylinder,cuz then the head has to come off!
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And the spring "keepers" are easy to drop and get lost in the crankcase..
since most motors need guide work anyway,and the valves might need grinding,and the head gaskets have 100,000 miles on them by the time it needs the valve stem seals,your probably better off pulling the heads and having them gone thru--but be aware many times when the valve job is done on an older high mileage motor,the motor often starts using oil because now the valves are seating perfectly,but the old rings will allow "blow by" to occur that would not have taken place when everything was a little "leaky" before..
.I've had more than a few "valve stem seal" jobs turn into complete rebuilds--sometimes its better to live with a few puffs of smoke and let it be--I figure at least the engine doesnt suffer from "dry starts" if it has a little oil in the cylinders when it first starts(at least the cylinder(S) with the loose guides and worn or missing stem seals....as long as it runs ok,and doesnt foul plugs,I'm just going to ignore the sometimes embarrasing puff of smoke when I fire it up!!..
