Glad I could help someone out here!
As far as the Oldsmobile 307 goes, I think that 1990 was the last year it was used.
Cadillac used the 350 Chevrolet for the Brougham from '91 through '96, and of course it was in the Escalade as well, until the LS engine came out.
1990 was the last gasp for the Quadrajet in any form from GM, unless it was used in export production models, but I don't have any information on that.
It would have been nice to know back then, I'd have bought a few for spares.
Rick