Hi, I was typing up a response when the board went Blooey! SQL errors, scary!
So, here goes:
A "Jerry Cam", (or cam from a '94-up 2.6), Pacesetter exhaust header, and larger, less-restrictive exhaust (say, 2-1/4) with a good turbo-type muffler such as a Dynomax Super Turbo, will wake the ol' 2.6 right up.
The old-style catalytic converters are quite restrictive and you'd do better by installing a generic Catco cat (very inexpensive and a good brand).
We have also been talking about throttle body mods in this thread, if you install the newer-style upper-half of the intake along with the round, freer-flowing throttle body, you add even more performance:viewtopic.php?f=22&t=97314
An older thread with more good info:viewtopic.php?f=1&t=78730&p=695726&hilit=chamber#p695726
If you really want to go Nuts, find yourself a '94-up 2.6 head, which has smaller combustion chambers. When bolted up to an earlier 2.6, there's a boost in compression that would work very well with the other upgrades.
Far as the V6 swap goes, you'd probably be better off finding a scrap 3.1 pickup (only came in 4WD) and using all the parts from that rig. You'd have to change out the transmission, maybe drivelines, all accessories, exhaust, adapt mounts, etc etc. The biggest task would be adapting the EFI & all the supporting wiring & electronics. Completely different from a 2.6.
Use a rebuilt, bored-out 3.1 (which will result in "3.2" displacement) or a Camaro/Firebird 3.4 ('93-'95) for best performance. A junked, rusted, or wrecked 3.1 4WD Rodeo would do as well for a donor rig.
A few more links:
"Jerry Cam" developed by our resident Isuzu Meister Jerry Lemond: viewtopic.php?f=22&t=79422
Pacesetter Header: https://www.summitracing.com/parts/psm- ... make/isuzu
Not cheap, but you probably won't like the considerably less $$$ painted item, once all the paint burns off and the header starts to rust!
Dynomax Muffler: https://www.amazon.com/Dynomax-17747-Su ... B00029J3IOhttp://www.ebay.com/itm/Dynomax-17747-A ... Sz&vxp=mtr
Catco Cat: https://www.summitracing.com/parts/cto-6575/overview/
There ya go, something to chew on! Enjoy.............ed