what gauge box tube to use for my custom rear frame project

[tranz am]

I'm looking for suggestions on 2×4 box tube thickness for my custom rear frame project. I could go with 1/8 or 3/16. I would like to go 1/8th because I can get 10 feet of the stuff for $40. 1/8th seams real close to the frame thickness. The stock frame measures 1.40 vs the 1/8th @ 1.20. my thoughts are I will be fine using the 1/8th boxtube since it is boxed. Thoughts on this?
the frame I am working on is in my build thread in my signature. Thanks