I don't like undercoat. For one thing, the places most vulnerable to rust are places they can't spray it. Another is, one it lifts an edge you have a place for the corrosives to get in and not get back out. It can actually cause rust. My '91 was Zeibarted from day-1 by previous owner. It was just starting to rust in '06 when I parked it.
The best thing to do is spray used motor oil up under it in every nook and crannie and let it drip. Messy? Yes, EPA approved? Take a wild guess
. A good wash out when you can and spray some more is good during the season, too.
Anyone claiming larger tires have any noticeable negative effect are old wives telling tales. My experience is living proof that is garbage talk not worth the breath used. I'd say it's safe up to 35" anyway. So maybe you'll get !20k out of a bearing rather that 30k. Does that really matter? It matters more how you upkeep your vehicle. Cleaning and lubing annually, which very few do, will yield longer service life with large tires than stock tires on a truck that isn't done on. And this is coming from someone who makes a living out of trucks that are carrying loads throughout their lives. My '92 K3500 had 335k on it when I parked it. I ran 285/75s or 255/85s on it the whole 22 years... and that one I never serviced the front bearings on. I never touched a thing on that front end other than idler arm about every 80k, which I understand is a common.
I've run Squarebodies way up near 200k with 33-35s their whole life and that was on 10" wheels. 8" like they do these days would certainly be easier on things... not that it seems to matter in any noticeable way