Sandcaster:
The cover over the transmission hump is called a dog house. The dog house of both the high hump and low hump style, is attached with bolts to capture nuts on the under side of the floor board. The low hump will have a smooth, mild curved shape covering the opening, used on column shift transmissions (automatic & three speeds). The high hump dog house is nearly vertical and squared at the sides and rises approximately 6" and flattens off at the top where the floor shift tower pokes through the cover, used on floor shift transmissions (granny low 4-speeds).
I have a few of each. Also, post a topic in Truck Parts Wanted for a low hump dog house for a '60-'66 Chevy truck. This dog house cover will bolt into your cab utilizing the factory capture nuts, therefore you will not have to cut and weld in a donor dog house cover that does not fit your truck. Below is a picture of each.
Low Hump Dog House Cover High Hump Dog House Cover