Gas tanks - doesn't matter, but if you are replacing, go with a BC Broncos 23 gallon tank. The extra capacity will let you get rid of the saddle tank if you want & the mounting system is better than the single strap that everyone (and original) employs.
Steering shock/stabilizer - any shock will work. Most of the time you will want to match it to the shocks you have on the axles.
Gauge cluster - Like Bob said, go digital. There is an all in one headset unit that fits in the original opening. BC & Wild Horses sells it.
Brake lines - Go stainless braided lines for your flex lines.
Rear Disc Brakes - The BC Broncos kit is the absolute easiest to install & maintain. Doesn't take long to do at all, about a day from start to finish.
Seating upholstry - Try LMC Truck. They sell quite a bit of OEM type equipment
Can & tire carrier - if you plan on going with larger than stock tires, get a frame/bumper mounted combo rack. If you put anything bigger than a 30" tire on the original rack, you will start to see body separation in your tub lines because of the weight of the tire. Check the vendor warehouses.