I was talking with Al Zim (owner of Zim's Autotechnic) and he clued me in on what those big round holes in all the floor panels are for!! Seems that they used a very similar setup on the 356, and he had seen them in use.
When the tub was built, they mounded the various pieces to the jig with a twist lock setup. Then the rest of the tub was assembled to those pieces. When they were done, they pulled the tub off the jig, and installed the plugs into the panels.
He even showed me pictures of a partially assembled 356 tub on the jig, and he showed me the same plugs in the floor of one.