Probably a stupid question, but if I'm swapping out 1" threaded headsets and the new one has a shorter stack height, can I add some spacers for 1" threadless headsets on top of the washers (or instead of a lockwasher) to make up the height rather than cutting down the fork? We'd be talking 10 mm or so here, not a huge stack of spacers or anything.

While this may work I would suggest you get some threaded spacers which typically come in 2mm width give or take and can be had at pretty much any bike shop or coop fairly cheap.

Worked for me on my touring bike. I think I picked up a set of CF spacers from Nashbar. I installed it in between the lock washers.

Brad

Ok, I have another, related, and probably even more stupid question. Are fork crown races interchangeable between sizes? Like if I have a headset for a 26.4 mm fork crown race and my fork is for 27.0, can I just buy a different fork crown race and use the rest of my old headset? Or does that not work because everything will have a slighly wider radius too?

These questions are because I have an ISO (26.4 mm fork crown race) headset and I want to get a new fork. The current fork has a ~220 mm long steerer (well my SAE tape measure called it 8 5/8" which works out to be 219 and some change mm), which apparently is an odd size for an old 26" mtb with canti brakes. I finally found a fork that looks like it'd work, but it's 230 mm long (hence 2 5 mm long spacer or 1 10 mm spacer if I don't want to cut, I was looking at these because I couldn't easily find washers for threaded headsets) and I'm still waiting to hear back from the seller if it's JIS or ISO with regards to fork crown race sizing.

As long as the stem is long enough into the steerer tube you'll be fine, but I would still use the lockwasher.

By into the steerer tube, do you mean past the threads? I was planning on using a Nitto Technomic that's pretty long, but I'd been hoping to find an even longer stem (or one with a shorter threaded length) so I could have more of the stem exposed. This is a mtb to drop bar conversion, which works best if you use a size or two smaller frame and then a tall stem and seatpost to make up for the fact that the top tube is long relative to the seat tube (in my case it's a 22" C-C seat tube and a 23" C-C top tube with a really long seat tube). Previously I used a Soma Hi-rise threaded to threadless adaptor to get the bars high enough to work, but I'd like to use a threadless stem like my nitto if I can make it work because I think it just looks prettier.

I had my touring bike's threading extended further down the long steerer tube..
and between the adjustable race and the top nut (about 2")
I have: a washer , the brake housing stop for the front brake, another washer,
2 thin threaded locknuts, tightened against each other ,
a tube sleeve with the base of a bell brazed to it. [rest of the bell did not get
powder-coated] and the top nut [Tange , with a stem O ring ]

I was planning on replacing the brake cable housing stop with a washer I already have that's the same size and going with a fork mounted cable stop because I'd heard they provide better leverage. Is that not the case?