The '75 definitely came with a folding frame and top as standard. A previous owner must have replaced it with a stow-away frame, possibly for the added space obtained at the rear of the seats. I seriously doubt that you would have to drill and tap for screw, everything should already be there, if not, then the holes shold be there for the brackets. The roof originally fitted should have the fixed rear window, the zip-out window being introduced in 1976.
The brackets for the stow-away hood (I'm a Brit, so for me the hood belongs over your head on a convertible. Roofs are for houses and saloon cars)
were fitted for the life of the MGB, being used for the tonneau rail. Your best bet would be to take a look at another MGB to see the way that the fold-down hood is installed. You may even be able to pick up a hood and frame second-hand, then you won't have the hassle of installing it.