Congratulations on your recent addition; she sounds like a real sweetheart!
I'm excited for you and your husband and hope we can help make your kitty's pregnancy a little less stressful for her and for you.
I'm kind of surprised that your local SPCA doesn't spay/abort, but I suppose, if their policy is to not fix cats until they're adopted anyway, it wouldn't matter whether they had a policy in place to deal with unwanted pregnancies or not.
Some cats start to seem restless when they're nearing the end of their pregnancy. Not all cats exhibit all of the signs or behaviours of pregnancy, but if she seems to be panting and/or pacing a lot she's probably within a couple days of giving birth--she may be looking for a warm, dark, quiet place to make a "nest" where she will have her babies. I typically recommend that people help out by creating a couple of areas that might appeal to a cat that's nesting because it will help you to keep an eye on your kitty if you know where she is and will allow you to better control where she decides to nest, provided she's happy with your chosen location(s). Some cats find less than ideal places to nest in--closets are a favourite spot, but the closet in the spare room is probably much preferable to a closet in, say, a young child's bedroom. Some cats also seem to completely vanish because they've chosen an obscure or hard to access location for their nest, which makes it hard to monitor mommy and kittens and makes for a stressful experience for the cat owner. It would be ideal if you were able to provide a "safe room" in which you can isolate your kitty and provide a suitable nest away from things that might stress kitty out--keep in mind that, if you plan to rehome or surrender the kittens at 8 weeks, mommy and kittens will likely occupy this room for most of the time they're living with you. Young kittens can get into a lot of trouble once they become more mobile (around 4 - 5 weeks), so they should never be outside of their kitten-proofed
safe room unless you are there monitoring their every move. Once the kittens are older, and escaping the safe room becomes a possibility should the door be left open, a baby gate in the doorway that mommy can jump over but the kittens can't scale is a really good way to protect the kittens from getting out of the room and into trouble, while allowing mommy cat to come and go from the room as she pleases (provided that you don't have other animals or young children that could potentially access the room as a result). A safe room with a closed door (or a gate once the kittens are a bit older and mommy is a little less protective of them) will reduce mommy's stress and will prevent her from moving the kittens to other locations within the house herself--which some mothers will do if they become stressed or feel that the nest has been disturbed or is unsafe. It's important to keep young children and pets away from the safe room, again, because it's really important to prevent mommy from becoming stressed.
There are a lot of things I could and maybe should mention about kittens, (such as the fact that, for the first couple of weeks, mommy will need to feed her kittens approximately every 2 hours, round the clock--and what to do if she doesn't), but there are entire books devoted to raising kittens, and I don't want to overwhelm you with a deluge of information that may or may not be applicable to your cat and her kittens. If you have any specific questions though, I or someone else on the forum will, I'm sure, be happy to answer them.