Not a simple job by any means, but *possible* (hey, anything is possible with enough time and money). If you're *very* good at bodywork, welding, etc, it is a task that could be done.
The overall size of the rear area is about the same and the width is close, but the superstructure (underneath) is somewhat different. The leaf spring mount area is completely different, for example.
Though, I'm not sure why anyone would do this?
I'd hate to ruin a save-able Frog to do this (in fact, if you find a Mk 1 with a wrecked front section, you'd be better off to weld a square-body subframe onto it and add a fiberglass Bugeye nose).
For the record, you can buy the front and back sections of Bugeyes and square bodies in fiberglass from various vendors.