An inline filler. Who'd'a thunk it? It'll do. One problem solved.
The guy who does that site is well-known in the Fiero community for engine swaps. He knows what he's doing; if that kind of solution works for a 4.9 Cadillac engine, it for damn sure should work with half that much engine.
The store connected to that site also offers a service: they'll custom-make AC lines for your application. Send them the ends, a sketch of what you need, and $125, and they will send you the AC lines.
My one worry was how to get air conditioning in this car--would I have to leave it off? The GM compressor will not mount to the Ford engine, and it was the height of foolish optimism to hope that the GM lines would bolt to the Ford compressor. This will solve that problem. I was going to convert the AC in that car to use R-134a anyway, since R-12 is illegal, so I don't have to worry about much else in that department. (Except, of course, that I'll need to get an AC compressor, since the one for my engine stayed with the car.)
I still have lots to do, but I'm glad to see that doing some thinking, and research, is yielding some important and useful information. I never would have thought of an in-line coolant filler, for example.
More and more I'm coming to the conclusion that the single most difficult part of this swap will be the shifter mechanism. If I can just make that work, the rest will only take parts and effort and time.