Jonesing for some 1" box.

How much cross bracing should I be putting in the wagon before I cut the bottom out? 

I'm thinking a big "X" from roof level at the C-pillar to just above the rockers at the A-pillar and a couple diagonals front and rear. 

Please tell me why I'm wrong. 

A bunch.....X at the 'b' pillar with diagonals to the 'a' pillar rockers...and another at the 'c' pillar with diagonals between these(b&c) and two diagonals  back to the the tailgate opening...

Thinking out loud.

I think you may have an issue relying on the pillars to keep your X triangulated, and would want a horizontal bar keeping the distances between the rockers accurate front and rear, in addition to what I imagine your diagonals to be; I'm not sure I'm interpreting you correctly, but I imagine one from A-pillar top left to front of right rocker, and a similar diagonal C-pillar to rear of rocker.

I am not an authority, but I like thinking about structures.

It's only 7.5' from the roof at the C-pillar to the lower A-pillar where I want to tie in. So with an "x" from each side of the upper C-pillar to the opposing lower A-pillar the longest span will be less than 4 feet. 

I should mention I'm also going to have a bar going straight between the A-pillars about 6" above the rockers that will get used as a jacking point. So they should be located pretty well with a "Z" structure. 

Maybe a "Z" at the C-pillar also? 

Edit: I'm not removing the lower rockers/sills, so they should help with triangulation too. 

Being able to get into the cab for cutting/fitting/welding is going to be important so I need to keep the number of bars to a minimum.