Clay;
The key questions are how much unsupported length you will have and is it necessary to bend it or is the belly pan flat?
If it is flat, Alclad 2024-T3 is what I'd use; it's very strong and the Alclad coating gives it good corrosion resistance but a coat of zinc chromate on both sides will be even better. If you need to bend the panel, then half- hard 5052 is good.
If you have some stringers, etc., you can get away with pretty thin stuff-- 0.049" or 0.063" but if it is not well supported then something like 0.120 or more might be needed. Weight isn't a big consideration in LSR but thick aluminum gets expensive.
Aluminum isn't the only choice-- there is mild steel, stainless steel, or (choke) titanium.
Regards, Neil Tucson, AZ