I have a 91 LS6 in my 79 Z28. I believe it was the last or next to the last year for the crate LS6. Mine is the p/n 366250. List: $4,621.88 Purchase: $2,990.62 1/8/1991. As far as the value today, I'd take a look at what it costs to replace it and go from there. Probably a little more because it's an "LS6." I'm not an authority though...
GM Catalog LS6 Specs by
Dave H, on Flickr
LS6 PN Description by
Dave H
Not exactly right out of the crate...
20171017_175306 by
Dave H, on Flickr