And for some of the same reasons as Dave mentioned you'll want to fit the body, fuel tank, grille/rad, and hood if you're using one to get all the gaps good on doors and decklid as well as nose. Also possible you'll have to do some clearancing or such on the underside. Very likely you'll need to do some body work. Granted it was almost 15 years ago, but the last Downs body I was involved with took a lot of body work to get nice. Even if you don't need a lot of body work, you'll still want to take 80 grit abrasive (after liberal use of wax and grease remover) to all the gellcoat and put some heat to it (while bolted to the chassis) to let it completely off gas. I prefer to set it out in the sun for a few days, but where you are that's not likely to work this time of year. The main point here is you want to get all that "heavy" work done before putting final finish on the chassis.

As for the seat, I like welding up a flat tubing grid that fastens at the outer body mounts and spans across the whole floor. For added benefit I use panel bonding adhesive to join it to the floor (sometimes called Corvette panel adhesive).

Gennie shifter changed hands last year, so past experience may or may not be meaningful unless it's through the current owners.