I just bought a 98 GTP. I am very happy with it, but I was wondering if I go to a 3.25 s/c pulley what do you recommend as far as other upgrades to avoid getting knock?
I'd go with a colder spark plug (NGK TR-55s), add a trans cooler while you are at it.
Usually with anything less than a 3.4 you are going to need some type of cooling. Several options on this. You could get a Thrasher intercooler (www.thrasher-ep.com), add water injection (I used an Auqamist 1s but there are other options out there).... May as well get an appropriate sized SC belt too (to prevent slipping).
With a 3.25 and no cooling you will probably get too much knock and not realize the full potential of the mod..
If you check the ClubGP.com board, most people running s/c pulleys smaller than 3.4" have installed an FPR to allow them to manually adjust the fuel pressure to reduce KR.