I wouldn't go through the trouble of changing the rails. You need a puller to get the pins out and you need to remove the alternator and P/S pump. The rails aren't the problem on these cars. The chain stretches and the whipping action will break off part of the inside rail on the left bank( driver's side). This will cause the cam to stop while the pistons keep going up and down which equals bent valves. If you do a search, this topic has been covered. I would pull the covers, replace the oiler hold downs, adjust the valves, and inspect every lobe carefully. Be sure to pull the coil wire from the dist and ground it if you are going to bump theengine over with the starter. Valve seals should be replaced on these engines at around 100 to 125k. If you wait until 150k or more, the valve seals can start riding up and down on the guide and wear the groove so they won't hold a seal anymore.

Good luck,
Peter

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