I had one of those MPC 4x4 Dodge kits. The tires were 1/20th scale street tires. The Dessert Dog Offroad tires on the Fall Guy pick up will fit on these rims. My only worry about sleeving them would be the centers would be too large. Craig
You could probably add a small piece of red sprue from another kit to fill the space between the 68 lens and the 72 bucket. You can also get them from the Modelhaus. They have them for the 69 EC $2.50.
With regards to the tail lights, they are different. Look at the black 68 in the picture. You'll see small pieces of trim that connect the tail lights to the bed trim. The 69-72's don't have this. GM added reflectors to the rear and widened the tail light bucket to accomodate them. On a 1/25th scale model you may not notice the difference, but on the 1:1 you do. The front fenders on 70-72 Chevelles and EC's are different. The Chevelles have a body line in the middle.
If I recall, the AMT kit is based off the F-350 bed. The wheels wells on these trucks were moved backwards so the truck could use the tow truck frame. The rear ends were narrower than the F-250's; that's why most of the time you see these trucks with wide steel wheels in the rear with 33x 12.50 tires.
Based on the fact that the rear weels are located more to the rear, maybe this is the same distance as the short bed. You'd only have to cut the front of the bed to make it look correctly.