[Zane M]

Short of not driving it during inclement weather, you just need to be religious about washing it as soon as possible after driving it on the wet, salty roads.
Unfortunately you can virtually be guaranteed of developing rust. There are a lot of nooks, crannies, and generally areas of poor drainage on these trucks that trap salty residue and eventually rust. Usually from the inside out. My Grandpa fixed rust/repainted his '73 every 3 to 5 years.

I believe Utah's smog numbers are based on the federal standards. If you are passing California tailpipe emission standards you should not have any issues. You may need to tune your carb however, due to the change in elevation.

You don't have to have any "certified" replacement parts, but all of the emission equipment that was on the truck originally has to be there.
So if we are talking about the '81 in your signature, you will need the EGR, AIR, and a catalytic converter.
You used to be able to find shops that would overlook the visual if the vehicle would pass the sniffer, but the state has cracked down and there are fewer and fewer shops that will look the other way.
The seal on your gas cap(s) also have to pass a seal test.
As far as mudflaps, while I am sure there is a specification(and I don't really know or care what it is), the reality is you probably have a 50/50 chance that they will say something about it. I have had some places not say a word about my truck('87 V20, stock height, 265/75/16 tires on steel wheels) and others say I need mud flaps. I ended up putting flaps on mine because I do flip a lot of debris up on the body. I would suggest just giving it a try without, and worst case you fail, put some mud flaps on for inspection, then remove them(if you want) once you have passed.
Welcome to the state!