As I am finding out with my first truck build, an AMT Ford C series cab over, that AMT trucks leave some to be desired as far as parts fitment and related issues as I have read here and elsewhere on the internet. If you have built an AMT big truck with out any fitment issues, congratulations your one of the few, as for the rest of us......
I am going to break this up into parts, hopefully making it easier.
PART 1: Squaring the frame
The frame is the foundation from which a truck is build, much the same way a sturdy foundation is from which a building is built. Since the frames consist of two separate side rails and numerous cross members is up to the builder to take the time and make sure that the frame is square, if not then pretty much everything will not be in proper alignment from that point forward and make for a frustrating building experience.
Everyone has their different ways as far as holding the frame sections in place while their being attached...if it works for you use it....personally I use a flat,steel building plate and magnets along with modeling clay. Use only the two side rails/frame pieces and the front most and rear most cross members. Use the rear cross member that goes between the frame rails and not the cross member that attaches to the body frame (above the truck frame) as in the trash truck, city delivery/Coke, stake truck.
Locate a point on your frame that you can use as a reference..in this case I use the rear most locator pin on the frame that is used to locate the trash truck packer box to the frame. There is another one on the opposite frame rail same location. On the front I used the center hole on the frame half of the cab pivot/hinge. It's important that you do a diagonal measurement as pictured, it needs to be an equal length on both sides
Once you are satisfied that the frame is square and true, attach the all cross members except for the transmission cross member/mount and cab support despite what the instructions may show. You will locate and attach those while doing the engine and cab alignment.