I suspect it'll pretty much negate the purpose of doing the swap (from the fuel economy perspective). Have you or anyone ever done a gear vs. fuel consumption chart for that drivetrain? eg:
( from:
Showdown: testing RPM vs. MPG at a fixed speed - MetroMPG.com )
If anything, you should be gearing the V6 drivetrain to spin at even
lower RPM than it did in the Olds, since you'll be pulling less car around behind it. That would be the only reason I would attempt this swap, personally!