PB Blaster has been good to me. Let it soak in overnight and then spray a little more on when you want to give it a shot. It'll stink up your garage though. Joel's suggestion to tighten a little has helped me in the past, too.
Big long breaker bars or pipes slipped over ratchets may work in some of the bigger applications like suspension bolts (been there) but don't do that stuff on the smaller bolts - you'll just break off the head and then you'll have bigger problems to deal with. I've broken too many dang bolts in my 15 years of C3 ownership. I think patience is the key in this situation.... Good luck.
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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.