I agree with those who have commented above. Give her a box and some blankets to nest in if she'd like. She'll probably find a secluded spot to have her kittens, and you may not see them for a couple of weeks. Just let her and her kittens be for a while. She will be protective of them, so it's best not to go looking for the kittens after they're born.
Give her a warm place to nest and have her kittens. Try to give her canned as well as dry food. Some recommend kitten food as a good option for a pregnant cat because it promotes growth. It wouldn't hurt to take her to a vet for a wellness checkup if you can catch her.
The advice others gave you is great. If she's letting you pet her, over time, see if you can pick her up. If she lets you, a vet visit would certainly be a good thing. And neutering her after the kittens are weaned (as well as neutering the kittens) would be great.
We've always had barn cats, and they often end up being house cats. Luckily, my 5 dogs love cats. right now we have 2 house cats (both former barn cats) and 2 barn cats.