I would love to find a class 3 hitch, use that for bracing. But finding one for an ElCo is proving to be tuff.
The wagon and Elco share a frame design. You can fab one yourself from the plans in this post:
https://gbodyforum.com/threads/g-body-wagon-class-iii-trailer-hitch-plans.56222/
I'd suggest you consider replacing all the fuel lines and hoses as well, and if you ever think you will go with EFI, add a return line now while the rails are accessible. If you are likely to have to comply with emissions, don't delete the vapor return line.
Sand and paint the under body to rpotect it from future rust. At a minimum, do the areas that will be inaccessible once back on the frame. Since no one will see this and it will be out of the sunshine, you can use POR-15 or a number of Eastwood rust-converter products. Since you did not mention trouble with removing the body bolts, consider spreaying some rust converter up into the body mount plate pockets to offset any drama lurking in the there. I don't have good advice on how to do that, but Eastwood does make a frame paint kit with a 2-foot flexible wand for just such irritatingly inaccessible locations. If you did not clean out the insides of the front and rear frame rails, take the chassis to the car was and use the blast wand to flust and sandblast grit and junk out , then use the Eastwood stuff I mentioned above to treat the innner surfaces. The stuff is really runny so it flows well...you'll need a helper with paper towels and gloves to wipe off the runoff so you don't spoil your new paint. You might also put soem of this stuff into the rocker panels under the doors - I think there might be drain plug you can pop out. Consider putting some anti-seize on the body bolts when you thread them back in to protect them from future drama.
Paint the radiator support. It's a common rust location. So are the inner door bottoms.
If you don't know what will be your drivetrain, consider adding one or two of these to your frame rails to expand you options:
I made up a bunch of them, but have not yet posted them on the Forum.