I use this bluetooth scantool... not sure who makes one for obd1.
https://www.obdlink.com/products/obdlink-mxp/
There should be data on the grill block for your car here on ecomodder, maybe someone will add in on it. Since your motor is as small as it is, you might not need the tube to be that big. You'd have to experiment with it to understand how it affects your motor.
I forgot to mention (I think...) the whole motivation for adding the tube was to smooth out the vacuum fluctuations in the intake manifold. The 1st attempt got me the big bump in the bottom end torque. Recall this motor is a 4.7L V8, smaller motors may be harder to detect torque changes by the SOTP. I was shocked at how much added torque there was on the bottom end. I already knew that flow thru the air box would give better upper end performance so it makes sense to do the 2.5" valve.
I have 2 aero mods on my plate for this truck, 1st being the 2nd air dam under the radiator. Did this with success on the Ram so it should do well on this truck. Then the 2nd is to make a tailgate spoiler that has the flow separation pointed at an angle back into the vacuum bubble that is there. The goal being to cut it down some to reduce that drag.