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Check your transmission modulator. Could be a vacuum line. Either way it is a quick/easy fix.

I usually roll the carpet back a bit, that gives a little more room.

Am more concerned about load ratings. Also GM cars were designed for P-metric tires. While speed rating gives some information, LT tires should not be driven over 65 mph (some allow 70 with an additional 5 psi), the construction is even more important. A "C" rated tire is 50 psi, D is 65 psi and E is boy howdy. Also a P-metric can be Light Load, Standard Load, and Extra Load all of which make a difference that was not a concern in 1988. BTW I also look at the Load Index (generally in the 90-110 range for a passenger tire) and the UTQG which indicates the tire wear: 200 for a track day tire and >400 for a street cruiser.

Agree in general if running tires and wheels with matching specifications to the OEM fitment. Many of us have changed wheel diameter, width and offset, plus tire diameter, aspect ratio and width, as well as type of tire, so I do believe a bit of custom tuning may be necessary. One item I have noticed over the years has been some lack of concern for the speed and load rating of the OEM tires. I don't remember the load rating offhand but the factory H speed rating is 130mph, which does effect the tire construction. I have seen occasional references to tires that are a good deal and maybe a long tread life but a lower "S" or "T" speed rating. Probably fine for a cruiser but would fall short of the factory design parameters.

As someone who works for a tire company, I would recommend sticking with the pressures on the door. It's more complicated than this, but generally, tires are designed assuming you are following those recommendations. Mostly it is about having the proper footprint for even wear over the life of the tire. Running the pressure higher will generally cause the tire to wear faster in the center, and lower pressure will tend to cause shoulder wear. Of course this also depends on having proper front end alignment as well as rotating your tires regularly. This is kind of over simplified, but you shouldn't go wrong by running what's on the door.