Keep in mind that the DOHC will use an entirely different computer along with different sensors and what not. While it will bolt in, it wont be so simple. I think you will find the stock 3.0 to have plenty of power for your aplication with a fresh and quality rebuild. Parts for the sohc 3.0 will probably be easier to get a hold of in the PI as well.

99 Gen 2.5, fixing blown head gasket
89 SWB- 33's, ARB Front locker, SR rear locker/axle, SR F brakes, winch, WST Offroad Armor all Around, 2.85 Aussie T-case Gears (SOLD)
Sold: (2) 95 SR's, 86 SWB, 90LWB, 91 LWB
-Can Change a timing belt in my sleep..