78 Eldo power steering pump bolt - how to loosen

I want to change my thermostat gasket - it leaks. I read the FSM, foundd out that the AC compressor needs to be dismounted - friggin' engineers - thought that stuff didn't start till the mid 80s.

Anyway to move the compressor it appears I have to remove the belt. To remove the belt I need to loosen the 2 bolts holding the power steering pump in place. The outer one (closest to the fender) was no problem. The inner one sits behind the pulley with maybe an inch and a half of clearance - so no socket.

I found a long 9/16ths open end wrench, put it on the bolt. Bolt is really in there, would not turn. Soaked it with Blaster (waste of time probably, just soaking the head, can't get to the threads).

Put the wrench on again, really tugged on it, ended up rounding the corners of the bolt head.

Bought an extra long 9/16ths 12 point box wrench, which does grip the head. Still cannot budge the bolt, even with an extra 6" of leverage provided by a short piece of pipe on the end of the wrench.

Any ideas? I really do not want to have to take off the pulley, pull the radiator and condenser, and go after the bolt with a long extension and my impact wrench ....

thanks

Art