At the pulley you're probably dealing with, belt slip isn't likely an issue, unless you have an absurd amount of slack/stretch on the belt. I've worked on builds down to 2.55" without slip concerns, as long as the belt/pulleys are setup right/tight, without requiring any add-on bracket (8-rib conversions help at that point).
(Edit: a simple trick is swapping the smooth s/c 3.0" tensioner and 3.5" idler pulleys for a little more wrap on the s/c pulley... After that, going to a 3.5" on both can take up some belt slack, if you have one sitting around. If you're having to fully crank down the tensioner to install the belt without it springing back much after, your belt won't be the issue on anything down to a 3.0" s/c pulley or even smaller. You can also run the Bonneville/Park Ave tensioner/coil bracket for more wrap, but will also need a newer L36 dogbone mount, which needs a hole drilled/tapped behind #3 injector that most L67's don't come with. I've gotten to a 2.7" 6-rib on my GenV with that.)
The sensor/gauge you're using doesn't sound like it's sensitive to low values, as mentioned in my first reply. 100 psi gauges aren't meant for this application, which makes me question some other things.
Also, are you scanning the car to see the MAP readings, not to mention monitor the overall health of the car trying to be beat on? 1-3psi is barely over 100 kPa by 10-15, while even a stock pulley should register 140-160 ballpark.