frame sectioning...wall thickness of new tube?

Well after some more exploration on my 50 shoebox, i found a nice size hole in the bottom of the frame rail that was filled with fiberglass matting and painted over. Its a section about 2 feet long so i have decided to just section the frame rail. I am going to add the a whole new section from where the tranny crossmember is, back to where the frame starts to curve upward to go over the axle. my question is, im using 4x4 box tube to match the existing frame, but what thickness do you guys usually use for this stuff? I was figuring on .125 wall or .120 wall. I am adding some more power to the car, not the flattie, i have the 5.0L mustang motor around 300 hp or so. I think that 3/16 will be a little overkill, but what do you guys think? thanks

 

1/8 inch should be fine. Mic the frame in a solid ,clean spot and see what that is.

 

im using 1/8" plate on my 46' frame for boxing the side rails seems to be working out well using 3116 rod stick welding

 

if it where me, id be using 3/16"
but whatever you use, make sure to fish plate and gusset it well.