System 6 vs System 11. The overall dimensions might be close but you will have to do some modifying. Bolt on/off head for Firepower, hinged head for Pinbot. Do they have the same neck 'pedestal'? Wood slides for playfield on Firepower, pivoting playfield for Pinbot. The front playfield hangers that go into the lockdown bar receiver could also be different.
By 'everything', do you have the correct transformer for pinbot? Correct coin door switches? The power box? The lower cabinet wiring harness? Those items are certainly going to be different for a system 6 vs system 11 game.