How to box my 55-57 frame

[Tri5Nerd]

I'll be boxing my frame in the next few months, I have access to all needed tools at school.
The frame already has a early 70's camaro clip, and I'll be installing bags all around. Already have Shockwave to install at all four corners with a Heidt's 4 link on a narrowed 9" rearend.

I want to replace all the cross members out and notch and box the frame, I've seen this done on other year trucks with tubes.

Please post the type and thickness of tubes to use, also post pics so I can get some ideas of how to really do this correctly one time.
I'll pushing ~500RWHP and torque with this setup thus it needs to be able to take the punishment.

Tri5