[chris vickery]

If I was doing this up, I would probably look at sandblasting everything bare even before attempting welding her back up. Another key thing about a reweld job like this is to also take into account the gap or amount of material that was torch cut out or that needs to be removed to facilitate decent welding and to allow for this when welding the frame back as well.
What I am getting at is that you need to keep the mounting holes aligned between frame vs body as it would be quite easy to stretch or shrink the overall dimensions on one or the other.
I would be inclined to make the body fit together really nicely and then the frame can be adjusted to match and be unseen.
Should the weld gap end up being too large to effectively weld in a couple passes, the welder can always affix a backer plate on the inside which can be cut off afterwards. A large gap like this can be filled by "weaving" the weld or layering in multiple passes.
Considering the fact that they left the escape hatch uncut, it provides a great reference for how close the two halfs of the hull should be for welding.