I had the same issue about a year ago. I have a late 70s boat as well. I tried all the tricks but finally wound up removing one of the corner castings so that I could remove the tensioner bar. Then I had to loosen the striker with a large pipe wrench (wrapped in a towel to protect the steel) finally a rubber mallet and a chisel broke the stubborn thing free. After it was off I found that there is a block of wood that is inside the crossbar that swells and locks the striker in place. I assume newer boats don’t have this. Reassembly went much better. Good luck
