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The post speaks for itself. What a ridiculously good looking WD21, and I couldn't believe the price... https://bringatrailer.com/listing/1991-nissan-pathfinder/

My old 93 With the LT 31x10.50 15 was supposed to be at 26 psi cold. I run 28 for the street with my 33x12.50's. Do the chalk test to find the best pressure for your tire, wheel, truck combo. I would expect it to be in the 26-30 psi range.

In my experience, P420 and/or P430 have been failing cats. If there are other codes, I would resolve them first, but if no other codes, it has been the cats going bad. If you have an IR thermometer (non-contact type) the simple check for the catalyst health is to get the engine up to operating temp. Hold engine speed at 2000-2500 rpm for a minute. Let engine go back to idle and measure the temperature at both ends of the cat. A healthy catalyst will be at a minimum of 150°f hotter at the outlet than the inlet. If the temperature is nearly the same at both ends, the catalyst is worn out and not functioning properly anymore. If it is hotter on the inlet than the outlet, you have a restriction and the cat is bad. The upstream cats are the primary cats and the ones the ECU monitors for function. The downstream cats are reduction cats that are primarily for reduction of NOx. There are a few states that have California type emissions requirements, so finding California emissions across the country is quite common. Don't know anything about cataclean, but I am naturally suspicious of things like that being much more than snake oil. Catalyst failure used to be an uncommon problem. Used to see 1 or 2 per year, now I see a couple per month on average. Problem started getting noticable a couple years after E10 fuel became mandated across the country. The ethanol is not a good idea for our gasoline engines and does cause more than just corrosion issues in older cars and trucks. My 93 Pathfinder ECU is not programed for it. What it sees is a lean condition at the exhaust, so tends to richen the air/fuel mix to get the correct reading from it's O2 sensor. That causes fuel economy to go down and makes the catalyst work harder and hotter. Not good things in the real world. With the VQ engines, you have 2 O2 sensors that are after the cat, but the upstream sensors are actually fuel/air sensors. Similar to O2 sensors, but have a much wider measuring range and much more sensitive. Easy way to tell the difference is an O2 sensor will have up to 4 wires. An AF sensor will have 6.

I don't think the ratio is printed on the axle, unfortunately. AFAIK only the manual trucks had the 4.3 gears, so unless the donor was manual, you do probably have the right axle. You could check either by taking it apart and counting teeth or by turning the pinion to see how many turns of that equals one turn of the wheels. The limited slip means you won't have to check both sides like you would with an open diff (assuming it's still good). If you turn the pinion four and a half times, and the wheels turn a little less than once, that'll be the HG46 you're after. If it turns less than once, you've got the wrong gears. I have heard of wheel bearings failing a while after a collision. The other evidence you've found does suggest that the truck took a hit. Again, I'm curious what's inside your old axle. Nice trailer!