Unimog Club UK

Hi

I think one in the cab, or one on the bed just behind the cab will be fine.
As you say, round tube would be better, but the square section should be fine.

If you do it to follow the cab lines, it will look more at home, and like you said, suit the shape of the 411.
When you bolt it to the floor, use spreader plates, one on top (welded to the frame) and the other below, so that this sandwiches the floor, as the race and rally cars do.
Use decent grade bolts, and also make sure the floors are in good condition, no point in having a cage that will rip out the floor if you roll.
And lastly (sorry I appologise that I don't know your skill level) but make sure it's a good weld, I have seen cages break apart at the joints, (lack of good weld penetration)
A cage that falls apart can cause more damage to you!

I will be fitting a cage to my 404 when she comes home, I have use of my friends bending machine, that makes life a little easier.

Once you have done it, show us the results. 

Cheers