odd delema with a v belt

[scrappyk5]

there was a "v" belt on my vette that was there when i bought it and it lasted about 7,000 miles before it dryrotted and broke. i could not find someone who sells the "v & w " models so i had it interchanged with a dayco. the one they gave me , no matter how much i tighten it , it keeps stretching and breaking.
as if maybe it is too far down in the pulley , i dont know.
i looked at both belts and they look the same.

anyone have any ideas ?

 

[89GMCSuburban]

How much are you trying to tighten this thing. You should have a half inch up and down movement when it is properly tightened.

 

[dyeager535]

For awhile there were deep groove pulleys IIRC, I'd expect the wrong pulley/belt combo would act something like this.