Vert?

Do I need to brace the back of my cab before I cut the top off

No, not really. But you will need to do some various bracing after your done. Depends on if you are using a kit or possibly leaving it a permanent roadster. If you look on the 7th page of members projects for the Immortal Round 5 build there is a bunch of pics from my roadster build.

Ok cool I appreciate it it is going to be roadster with unibody full show truck

In that case, you will REALLY want to look at my build thread closely. When you close the doors after the roof is removed the cab corners shake a bunch. When I got finished, my doors closed as "soundly" as my other Mazda with a roof. Mine was a walk-thru so what I did on the inside will be very close to yours.

Something I will say / advise with yours being a unibody look, you will have to make a decision of the cab mounts vs the bed mounts. By this I mean the bed is solid mounted to the frame. The cab is rubber mounted to the frame. As you go over bumps, roll over curbs into the show grounds, or what ever... the bed will flex differently than the cab if you don't change one or the other.

Given the strength I added to the inside of mine, and the Mazda having a box frame, I would recommend mounting the cab solid to the frame. This will result in more road noise in the cab, but so what - its a roadster. By making the cab solid to the frame, and the bed solid to the frame... then the cab solid to the bed, you will stand a better chance of not cracking the outside surface where the cab is jointed to the bed.


This page will give you a better idea of the cab after cutting off the roof.


Near the bottom of this page you will start to get the idea of how I reinforced the cab, and then the bed.
For your install I would weld the cab reinforcing to the bed reinforcing.

^^^exactly what he said