It depends on how much you are going to remove I guess as to exactly how much bracing is needed. Under the floor pan area are cross braces. The body mounts are sitting on the cross braces. If you are not removing those braces and just replacing the tin floor, you might be OK to just cut. But I guess I am just nervous about body shift and keeping the door alignment in place. Years ago I began the restoration on my 73 Bronco and being a novice I did not brace as well as I could and paid the price by having to do a ton of work getting the door posts all aligned correctly. But I did ok in the bracing department and the truck is pretty true. But, instead of bracing I had actually hung the body of the truck from an I-beam by attaching beam hangers and then threaded rod down to some brackets I fabbed onto the hood hinge locations. Allowed me to remove/replace the front door posts without the body shifting.
I have seen people weld square metal tubing across the door openings and then whatever you think needed to hold the body ridged if you are going to remove so much metal in the floor that it might sag from the weight. You could even support with some jack stands from below or something. Thing to keep in mind is just whatever you think is needed to keep the body in place as you remove metal.
Again, it is a judgement call. Better safe than sorry, but if just doing the tin in the floor it might be OK to just cut it out without much bracing. If you are going to be removing the cross pieces that the body mounts are up against, then think about supporting the body in some way.
Another thing....take lots and lots of measurements. Like door openings. Across the truck from say kick panel to kick panel. Measure across the back. It all maters later when you are trying to obtain good gaps in doors and tailgate, etc. Pictures and measurements are gold later on. I had tons of measurements, which allowed me to get the truck back together after removing tons of metal.
I replaced ALL of the floor...
This... (this picture kind of shows how NOT to do it. I should have welded bracing across the bed. I had good measurements and had left the top on as I did all the floor, but bracing would have also been a good thing to do. This was some years ago when I considered myself a novice. Now I am somewhere above a novice, but no expert).
Became this..
