BOP th400's are the same with the exception of the short vs. long tail shafts. The "W" is a body code designation and as you stated came from a Grand Prix. I've got a 74 buick th400 in mine. Had to swap out the long tail shaft for a short and bolted right in. There are two types of vacuum modulators, one is stight out the other is bent. I beleive the cheby's are a little different (bolt holes?)but BOP are the same.
