George, there really is not a right or wrong answer to your question. It really is about how badly yours are damaged, and what you have to work with.
To my knowledge there is only one manufacturer of these, but I will also tell you that while they are good, they are not that accurate. Accuracy and authenticity may not matter, and if that is your feeling, then you will likely be happy with them. Also understand that most of the holes (for rivets or alignment) are not on the reproduction sills. On the reproduction cross sills, the provisions for the floorboard clinchnut screw is not there. To do them accurately, the area will need to be dimpled and then the D shape for the nut must be added. This allows the nut to set below the lip so the floorboards can be tightened enough to seal against the flange. The ends of the cross sills will need to be modified so that the floor pans will sit flush. The sides of the sills are not the correct height which will require some trimming. Also some reshaping in the corners to meet the quarter-lock pillar and the inner wheelwells.
It is definitely doable without a jig, but you are going to need to triangulate the body out well with tubing (electrical EMT works well for this) and make sure the deck door and the side doors all have the proper gaps before you begin. It takes a lot of patience and diligence to keep adjusting until it is correct. This is especially true for the sill extensions. If you need any pictures for inspiration, I can probably find you a few. Good Luck in whichever direction you choose to go.
