In my '59 Tradewind, the chrome piece in the tub floor seemed welded to the cast iron plumbing beneath the floor. I was unable to free it using a tool with 4 "arms" that engaged the chrome supports (spud wrench?), resulting in destruction of these supports. I treated the area several times with Kroil penetrating oil to (hopefully) loosen the parts. As a last resort, before sacrificing the floor of the tub, I cut two vertical slots into the chrome drain piece with a Sawzall, and used a pair of large screwdriver blades to rotate the chrome drain free from the cast iron P-trap below. It wasn't pretty, but I was able to remove the shower pan intact.