Outer Limits 4x4 Board

I am about to weld some holes found in the inner wall og the frame appr. 1 foot behind the spring bracket for the rear spring. Anyone know what material thickness to use (mm)?
Am i correct to assume that the outer, bottom and top surface of the frame is of thicker material than the inner side?

Natually, on one side there are some diesel tubes right across the welding area, and i have to bend them out of the way a little. Due to murphys law one of them of course had to rupture. Now, there are three lines there, one thicker than the two others. Am i correct to assume that the thickest tube is the diesel feeding tube, and the two others are feedback tubes?

one of the thinner lines is prolly a brake line.

No, the brake line is the fourth line, easily identified as it runs inward from the frame to a bracket and then turns to hose to the rear axle.

totto wrote:No, the brake line is the fourth line, easily identified as it runs inward from the frame to a bracket and then turns to hose to the rear axle.

You have used a trapping question, you didn't state four lines in the first post. Trace them down to find their source and destination. Diff breather? Fuel tank expansion breather? Factory diff lock?