OP, if you're talking about one of those stock 8" sealed enclosures, and SD-2 8" would work better, but make sure you check out the mounting depth and cutout size.
If the cutout size is too small you'll need to check if it's possible to enlarge it. If the mounting depth is too shallow, you'll need a spacer (which might mean that you'll have to leave the plastic cover off)
Probably your best bet for an SA-8 would be to build an enclosure. If you wanted to keep a factory look, and the SD-8" wont fit in the stock enclosure, you're going to have to measure out if a fiberglass enclosure would fit inside of that panel, and build yourself one.
If you were thinking of doing Infinite Baffle (free air into the trunk), neither driver would be optimal for an Infinite Baffle alignment. If possible, I would look into maybe doing at least a single 12", I recommend Fi Car Audio's IB3's. For inspiration, here's a good example of IB3 12's installed into a trunk:
http://www.stevemeadedesigns.com/board/topic/143276-2003-chevy-monte-carlo-ss-audio-build-ib-build/