Send Jerry Lemond at jlemond@bellsouth.net a note.
He retired from Isuzu after 23 years of being their head instructor. He deals in Isuzu parts now. He sells all factory parts and sells them at better prices that you will find at any dealer. He will hook you up with the correct plugs, wires, and O2 sensors that you will need based on the production date of your truck. Some of the 3.5's used Champion plugs, but I'm not sure which units they were... he would be able to tell you everything you need to know.

1995 Trooper LS auto 3.2 DOHC /w SOHC intake
1989 Trooper 2.6 auto
1989 I-Mark RS DOHC 1.6
1991 Stylus XS DOHC 1.8