Hallo hallo! We're renovating an 1850s stone farmhouse, and occasionally bump into issues which I can't find immediate answers for just by asking the great god google, and I thought I might find better answers here.

We pulled up the original floor planks in what was the summer kitchen, and pulled down the plank ceiling as well. This space will eventually become a proper kitchen (vs a seasonal one), and we'd like to be able to re-use those old boards if we can.

The ceiling boards are 1" x 6" x 16' or so, probably oak. They're dirty on the lower side from 200 years of smoke and domestic grunge, while the top sides are grungy from 200 years of dust, garbage, coon poop and worse (cat's pee).

The floor boards are heavier, probably oak as well, 2" x 8-10", 16' more or less, and cleaner, as they had layers of oilcloth (at least, I think it's oilcloth) and lino protecting them. There does seem to be the remnants of an old finish on some of them, but maybe that's just 200 years of wear, hard to tell.

We've pulled all the nails (hand-made, square-headed, like modern horse-shoe nails only longer), and I'd like to give them a really good cleaning before we proceed.

DH thinks we should pressure-wash them, probably not with too high a pressure, and probably with a detergent.

I think I should scrub them with a scrubbing brush and soapy water, then rinse and dry.

I'd then like to use them to floor the new space, so, not sure whether to leave them scrubbed clean and then finish once installed, or install and then sand and finish, or have one side re-planed before installation, or have them tongue-and-grooved and installed, etc, or.....? And, screw them down onto the subfloor (which will presumably be ply?) with nice-looking screws, or go the t&g nailed route, or...?

I see reclaimed or salvaged floors for sale all the time, so presumably we're not reinventing the wheel here, but I would like to get it right first time! We do have enough gash boards that we can experiment with different cleaning methods, and we're very open to suggestion.

So, my questions are: what is the best method for cleaning the boards, and then, once they're clean, what's the best method for re-installing them?

And then, once all that's done, I don't want to stain them, so are there any floor finishes I should consider, or avoid?