Before I got my air compressor I had a 1940 leaf spring bolt I couldn't budge even with a breaker bar. I bought a cheap
Chicago Electric 1/2" Impact from Harbor Freight and it did the trick.
I use an air impact in the garage now and just carry the electric impact with me for an occasional stuck bolt I might run into during work. If I used it very often I'd buy a better quality impact, but for occasional use it has saved my butt more than once.
Your problem sounds like having the clearance to use one, either electric or air. I'd invest in some good 6 point sockets, breaker bars, and as
yakky mentioned some good penetrants and a way to apply heat.