If you look at other welded frames and component videos and forums out there you will see that welding distorts a lot and this has to be corrected via machining or grinding. I suggest you design all bolted joints (to allow adjustment) or you braze the connections not weld them. Brazing doe not distort as no metal melted. Channel of suitable size is just as stiff as RHS in bending yet it allows you to bolt up surfaces properly. Channel is not as stiff in torsion but then your bench is not going to be twisted so matters little. Do a channel vs RHS simulation and see what you think. Regards Peter S