When we make out own flooring, like from reclaimed lumber, we use a slot-cutter router bit and groove all the boards, no tongues. Then we cut a bunch of spline stock either from scraps or 1/4" plywood. You have to get true 1/4" for this to work, not the somewhat thin sanded or lauan stuff, unless you invest in a router bit that's an exact match.
Anyway, why would we do it that way? you are conserving material by not wasting 1/4 or 5/16 of face on each board. On reclaimed material that is over $10 bd ft this can add up in even a medium size room. The setup is also easier, no chance of mismatches at the t&g (grade each board before milling and make a prominent X on the reverse, mill with router on the top of the board)
Butting the boards makes the finishing more problematic, and any seasonal movement shows gaps down to the subfloor, etc.
Casey

If you are on slab, yes you need a really good moisture barrier. If you are on crawl space, then the ground under your house needs the moisture barrier, and you need ventilation for the crawlspace, and insulation under the house.

Even with all of that, without museum quality humudity control in your home, you will still get seasonal shrinking and swelling. Winter will be the worst, with the gapping. In the olden days when finished floors were direct applied to the joists, that gapping would allow the dirt that drifted into the spaces between boards to fall through to the ground below. With the solid subfloors of today, over time, you tend to get that black dirt line between boards that even focused vacuuming in the winter won't quite get rid of. And you may get some permanant edge gapping from the edge crush that happens when summer humudity creates swelling in the wood that forces the edges together and crushes those fibers on the edge.

It will be glorious! Especially if you take the time that you have because you are DIY and finish the backs and sides before you install. That will help with the above moisture transmission.

It's wood. With all of it's plusses and minuses. You can't make it behave like man made plastic laminate. :-)

Im not super upto date on real antique styles of layin wood. But i would suggest some black felt paper to be layed first just so you dont see the subfloor if and when your boards gap. Or paint the plywood black.

Well, the black felt paper is a good idea. :)

My floor is a bit odd. It was a slab.
We raised the floor to match the other parts of the house, so there's a 4-5" (I can't remember and am too lazy to get up and measure right now) insulated area between the slab and the two glued 'n screwed layers of plywood I walk on.

Now, here's my thought, but I have concerns. Tell me.
I will probably not get to this until winter, so it'll be dryer in here.
If I were to lay it and leave, oh, say, 1/16" space between the boards, in the summer would that be enough not to cause it to mush up when it swells with the natural humidity? I know, all conditions are different, but am I on the right track:?

I also realize the whole old pine thing, expanding and shrinking and making cracks is unavoidable. I'd have to be more vigilant about actually vacuuming.

I'm thinking of plugs to cover nails, if I can find them.
Casey, I'll seal both sides and any cut ends. Good advice, as usual. :)

One thing you could try is making a routed square rabbet and overlapping the boards like they do with siding.

lazygardens, that's crossed my tiny mind more than a few times. 5/8" is 1/2 of 3/4", isn't it? Hmmm....
Got those two table saws....