You have the taller gears in the Buick so it will hurt off the line but should be good for MPGs.
My first question is what are you using to scan and program the car?
Have you calibrated the MAF?
I'm using a HP Tuners device. Pretty slick, but it's been hiccuping a bit lately.
The MAF is actually the larger Cadillac unit, so it was recalibrated when it was changed. And the supercharger is the later, larger version (I forget which "gen" it is, but it's not the '98 version)
And the Buick has the taller gears from the factory, but mine were swapped out for the shorter gears. The car used to do a lot of city driving. Now that I do all highway, I wish I had the taller gears back again. Besides, without racing slicks, the extra gearing is useless at the racetrack (actually counterproductive) off the line because of the traction loss, it would be better to have the taller gears and stay in 1st longer - I don't need any extra torque multiplication.
I tried it with my GTP, and just noticed that the car was a pig. That is about it. You could always go with a bigger pulley if you wanted.
I have 3 pulleys, but if the difference between the pulley I'm using and no belt at all is only 1/2 MPG, then I would think that changing from 3.4" to 3.5" would be useless. I'm actually thinking of putting my 3.2" back on, and I'm guessing that will have no noticeable difference.
I think my hardware has been on the fritz lately, I haven't hooked it up in a while. I was getting more and more partial flashes (wound up being stranded a few times), so I've stopped using it. It will probably have to go get repaired...