I have replaced both belts on my 88 250 w/460 and also the tensioner pulley. Belts are fine but pulley still squeals, any ideas, PLEASE????

 

How much tension on the belts when you installed them?

Just had to put new belt on my 302, though it wasn't really old, but the belt was stretched enough that the tensioner was barely pulled and allowed random slippage(squealing).

I marked the tensioner(with belt off), then once belt was on I could see how rotated the tensioner was.

 

I have a 351W and my tensioner comes apart easily, maybe just take it apart (it's a left handed nut on mine) and clean & lube it.

 

you might need to change the bearing in the pully... pretty painless job really

 

Either the belt i loose causing a squeal or the bearing in the pully is shot. They are cheap, like $15. Take the pulley off and spin it with your finger, if not really smooth feeling it will need to be replaced.