Fuel Tank Question

[Rifter_Chevy]

Hello all, i am new to your wonderful site but now that i own a chevy i will be around here alot im sure.

I just picked up a 1984 3/4 Ton 4x4 and have a few questions about fuel tanks.

It was originally a dual tank truck and has the switch and all the wiring for duals but current only has one 16 gallon tank installed on the passenger side.

I was wondering when i install a new 20 gallon on the drivers side will i be able to replace the current 16 gallon with a 20 gallon or am i going to find something that will stop me from doing this?

Also is there a good way to install Skid plates under the fuel tanks? Or perhaps a company that makes armored chevy saddle tanks for off road use?

EDIT TO ADD:

Its a long box

Thanks!