This is a great question... when our son was on his way ust 3 years ago, this was one of my first concerns.
I'm not sure the extent to which the laws are federal vs state, and may be different for you in OR than me in here in NC, but here's what I do.
I put the child's seat in the front passenger. Here's why.
The rear seat really is awefully shallow for a rear-facing carseat setup, and i never felt liek it would balance very well in the middle spot. I debated addin gshould straps to the side rear positions instead... however after trying to put it in once, I quickly reazied that the logistics of leaning back in there to get him into place required some serious body contortion. and just would not be practical.

I know here at least, ths law says you cannot have a child seat in front if (A) you have airbags, and/or (B) the back would be safer.
Since the rear seat is technically covered by fiberglass, not steel, and there is no airbag, one could easier argue that at least in a rollover accident, the front would be better in this vehicle. For side-impact, that's debatable when compared to the middle rear, but probably the same as side-rear.
Honestly, with the truck riding baout 6" taller than stock, plus 3/16" tube rocker panel protection that's right at height for most other car bumpers, I feel pretty comfy with him riding up there. Plus he LOVES being able to see the world when riding.