You have got the belt going the wrong way arond the tensioner

      Seems to be common with GM's around your model years.  Several people
I've talked to have had to quit accelerating so quickly.  One thing is for
sure: you don't want to use an el cheapo brand serp. belt.  Get a known good
quality belt and it sometimes will make it stop the acceleration-squeal. DO
NOT spray anything on it.  Ensure pulley-grooves are clean.
   Ensure idler pulleys(often made of pastic) are not worn excessively;
they tend to wear the outer edge faster and slower wear as you progress
toward the inner side(not square-across the surface, but at an incline).
   We've changed several of these, esp. on supercharged Buick Ultra's, to
both keep the belt from 'riding' off the idler pulley AND to reduce
accel.-squealing.  No adjustments other than replacing tensioner, as you've
already done.  HTH,  s