Yep, not a problem at all, with a couple caveat's.

The backbox itself, sure, will swap no problem, even back to System 3. As long as it's not a Hyperball style box with the big lower panel, but very very few titles used that.

Lamp board, you're probably going to have to move around some lamps or make a new one. Those are actually unique between games. Displays are all mounted in the same locations for system 7, but all the lamps and shields move around. Shouldn't be hard to make one though, can just use the photo from IPDB for the lamp locations, and any system 7 lamp board will give you dimensions for overall size and display locations.

System 7 PCB's are System 7 PCB's. Just a ROM swap and you're set (Assuming they're a working board set). Again, only Hyperball is the odd game out here. Some later system 7 boards don't have as many parts populated, but they all relate to backward compatibility, so not a problem for you.

I would get an aftermarket power supply instead of the factory piece, the Williams design has huge issues with the GI lighting circuit.

Sound board is standard system 7, I don't believe you need a speech board.

By 'Lamp Board' I refer to the white wooden board that hinges out, with all your GI lamps and displays on it. They're different for every game, you'll probably have to make your own. But if you get ANY system 7 game you can use it as a pattern to make a blank new one.