First off I'm going to say kudos for giving a stray cat a home. HOWEVER....
Next, I'm going to say that I completely 100% agree with Watchdogps and Lyonfuzz.
The reality is this: you have a semi feral to feral cat that is pregnant. There is no guarantee that she will stay in the garage with the kittens. She very well may move them and then you are looking at the very real possibility that she and all her kittens will not be able to be handled let alone altered. Leading to more unwanted cats that you cannot afford.
You also stated that you will spay her "after the birth as soon as we can afford it." This indicates that you cannot afford the vet bill if she were to have problems during birth requiring a vet. Nor can you afford to properly care for her or the kittens. Vet check ups, worming, vaccinations, kitten food (which she should already be on) to name a few. And as Lyon pointed out, it also indicates that you cannot afford to alter the kittens once they are born.
Having worked at and help start a rescue, I cannot tell you how many times I have heard similar statements and the animal never is altered and keep on having litter after litter because the money was just never there to have it done.......But maybe I'm just really jaded.....
Think about this: According to UC Davis, In seven years, an unspayed female and an unneutered male cat can produce up to 781,250 kittens. According to dogandcats101.com, an unspayed cat, her mate, and all of their offspring, producing 2 litters a year, with 2.8 kittens surviving per litter, can add up to 11,606,077 in 9 years. That is not a typo! 11 million plus cats in 9 short years.
MY 2 CENTS: If you don't want to have her litter aborted because of your beliefs then I have to respect your stance even if I don't at all agree with it. However, since this is the road you wish to go down, my advice is to take that young cat and release her to a shelter before she gives birth . They have the means and funds to care for her correctly and will have her and her litter altered. If there is a medical emergency, they have the staff to take care of it.
If you must have a barn cat there are many available that need homes and are already altered. Some are even free or are offered for adoption at reduced fees. Check with petfinders and your local shelter.