There is a Craigslist posting (Flint, MI. area) for a 86 Century Grand Sport (I know you said a 87). This may give you some indication of the values. I have heard of some problem from this dealership related to a title issue on a car. I think it had to do with a Salvage Title on a Regal & trying to pass it off as a Grand National. So do your due diligence if you deal with them. This Craigslist posting has been up for several months now.
Here is the link > 1986 BUICK CENTURY GRAN SPORT VERY RARE ONLY 78K MILES!!! - cars &...
I have a factory vacuum block. the molded hose connection (25522909) for it I found as NOS on ebay. link below...same as pictured on that craigslist ad.