I think the advice on pulling the heads and studding it is best.
My '86 has had a turbo on it (Banks Gen 1) since '87 and 7K miles. In 2010 I blew a head gasket and had occasion to tear into the engine. I saw the effects of turbocharging an NA engine and them working the engine hard over many years. I hate to think of how stretched and weak the head bolts and gaskets would hold up on a 285K engine if you weren't very careful. Head gaskets and studs first and foremost! I saw signs of fretting and long term leakage on the head and block gasket facings telling me I had lifted the heads on more than one occasion. Well, that is no longer an issue with the studs and freshly redone heads. The valves were really in bad shape considering. The bores, pistons and bearings held up well but my engine has a lot less than 285K on it. If it were me, I would budget the money to pull the heads and get them redone. Evaluate the bores while you are in there and the block deck. Install head studs, the reworked heads and the turbo kit while you are reassembling the engine (it will be easier that way... it's not particularly easy to install). You'll end up with a better result.
Or, you can install the kit onto the engine and hope for the best. If the engine is in good shape and you don't wail on it, nor turn the fuel way up, you "should" be OK for some period of time. Bear in mind that the turbo will uncover and magnify whatever weaknesses there are in an old, tired engine. All you can do is hope those weaknesses are minor and easily fixed. First and foremost, go easy on the fuel settings. Adding more air in itself is not the problem so much as the extra fuel you put in with it. Banks had two fuel settings and on the low (emissions) one, it really made the truck less smokey and EGTs were actually lower than stock under the same load conditions because of the cooling effect of the air. Didn't make as much power as on the high setting but it wasn't as ******* the engine and nowhere near as smokey.