Dave,
It’s really neat to see all of your parts during the build stage. I bought my car completed.
At tip of screwdriver, fiberglass tub (footbox) is inboard of the post. The glass is also inboard of the 60’ angle brace forward of the post.
At tip of screwdriver, tub is closer to body to form a recessed area for the fuse blocks which are hidden behind carpeting.
The glass in back of the post extends to and bonds with the body and goes all the way to the door sill.
I may do a heater in a future project. I assume the general use of MGB parts in these cars would include the heater.