The 46 bowfront is 36" wide correct? So you still can use your 30" fixture.
I would get new substrate, if you use your old one, it might release nitrates/phosphates into your new tank, which you don't want, and you can move everything, since your rock is already cycled, i see why you couldn't?
I would first add the substrate/saltwater --> aquascape --> add corals/fish
IMO