I tried the longer soak idea followed by heat but that failed. Also, evidently my little Milwaukee cordless impact is a bit too strong for this job. I thought it was loosening the bolt but it was twisting it and it snapped the head off.
'48 Zephyr, restored '06, runs great
'54 Fastwin, restored '09, runs great
'57 Fastwin, unrestored, locked up in 2018