Yes, you should change them. If you don't use your welder that often, your wire will start to rust (even in the high desert). Rusty wire is the death of liners. I have to replace mine almost yearly.
Also, those little clip on felt wiper deals will help keep the liner clean. Or paper clip and...
I feel your pain, there is no way I'm going junk yard hunting in Missouri in the summer. No matter how big the discount. Save that for the fall/winter. It's actually easier in the winter, stuff rolls/slides better on frozen ground. And the lack of blood sucking bugs........
Ryan should be able to get you any dim's you need.
Quick is relative, that's a lot of rod to throw down. A lot of tacking to lock it for welding. Your going to have to jump around a little to keep it from becoming a banana.
You want design flaw, you should see the semi loads of Toyota frames going through here to replace all the rusted out ones. Box frames hold way more water than C channel frames. Mix that with a little road salt and you're boned.
I saw a Canyon in downtown Dallas that was being used as a delivery truck. It had a lift gate on the back of it. The gap between the bed and the cab looked a little big. Looked like one more good load and it would look like this truck.