My 58:
I'm pretty sure I bought the weather stripping for the rear doors from Classicparts.com Their showroom was bout 20 miles from my house in the KC area. I was very spoiled in KC. Jim Carter was about 5 minutes up the road, Classic Parts was about 25 minutes and LMC was about 45 minutes if I was feeling motivated. I usually alternated between Jim Carters and Classic Parts. My truck club met in the Classic Parts showroom once a month so I'd usually load up while I was there.
Anyway, back to my point. I have clamshell doors on my Suburban. The weather stripping was kind of a universal fit that was just a long strip with the plastic plugs spaced along the length. If you have clam shell doors that is a good option. If you have barn doors I'd go with Bill's suggestion and get a set from Steele Rubber. I've never bought from them but they have a great reputation. I believe Bill provided them with a set of NOS or really good shape examples to make their "mold/pattern" from so they are supposed to be spot on.
Like Bill said, my spare mounts directly under the cargo floor just behind the rear bumper. There is a long hinged support bar with a key hole in the end that slips over a large round nut at the end of a long bolt that hangs down from the opposite frame rail. It should all be laid out in the assembly or shop manual. Quite often these pieces are lost/tossed over the years. I was fortunate that mine was complete when I bought the truck.
Your plan sounds good. Get yourself a set of weather seals and fix the doors. If that doesn't work, make sure all of your sliding glass weather stripping is good. If not, fix that and see what happens. As a last resort, reroute the tail pipe. My exhaust had to be done anyway, so I did that before the other steps. Good luck!!
