The only time I think the original layout is critical is if your restoring a unit with historical significance. Such as Goranson's 55 cruiser
or 66 Overlander's around the world unit
I'm staying with the original layout, using the original interior, but updating the bath and changing out the gaucho for a built-in dinette.
I can think of three issues (probably minor).
1. If you use the original interior outer walls, you'll have a lot of rivet/screw holes that are now visible due to layout changes.
2. Interior plywood walls were cut to fit the profile of the outer wall area they were attached to. You may have to re-profile the adjoining edge to fit.
3. Updating mechanical systems, some of the cabinets may have to be modified to accept the new units.
Good luck and enjoy the process.