Not an easy task, for sure. Just did this a few months ago on my '66. Pretty much ended up using a crow bar to pry the body up away from the frame enough to get to them and get them out. Need to make sure you loosen all of them on the side of the car you're working on so that it doesn't fight you more than necessary, and also so you don't have to "pry" against your car more than needed. I wasn't too thrilled about prying against the car, myself. I was lucky and didn't have too many that spun on me, but did have to cut 2 holes in the trunk to get to 2 of them that were too rusted to come loose. I was able to use a socket/ratchet from below and a wrench or channel locks to grip anything that was spinning on me. Unfortunately this is one of those jobs where "staying after it" will win the day. It's well worth the hassle though.