recommended rod for welding 3" thick steel

we have to build a box out of 3" thick steel but not sure what rod we should use, could you help? another thing is what kind of bevel should be done on the steel.

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Probably the biggest diameter 7018 stick you can handle for the position.

If the welds can be done flat or horizontal, a 3/16" rod will put down a lot of metal.

bevels should be adequate to allow for complete penetration if that is what the print or application calls for .
usually around s 45-60 degree included angle.

if you are welding to a spec, like AWS D1.1, the joint specs can be located there.