Im replacing the floors in my 67 chevelle, and the floor braces are rusted about gone. Do I have to support or brace the cars body in any way to prevent twisting when I cut the braces out. Are there any important steps I should now about before I do this. Thanks.

Hi Brian. There are a number of way to do a floor and braces.
This is the way I like to do it, if there is sever rust and floor brace replacement...
You must jack the body up off the frame, if it's not off the frame already.. It only has to be 4 or 5 inces so you can remove the old and fit and weld in the NEW..
Most floor pans do not include the drivshaft tunnel.. Thats good.. because you now have
three good spots to fit your new brace to that will conform to the orginial conture.. RH & LH inner rocker panels and the trans tunnel..
1st. Drill out and remove the welds on
ONE floor brace... clean the area that's not affected or being replaced on the above floor... Fit brace to the floor inner rocker brace and trans tunnel... Drill holes and secure with sheet metal screws... ( spray weld through primer on the top edge of the brace B-4 you install it
also drill holes to plug weld the end flanges to the inner rocker panel..) When fitted and screwed TIGHT to the floor, from the top (inside) you will have the old drilled out spot weld holes in the floor,, Makeing sure the floor brace is flush /tight to the floor
plug weld it in the areas that YOUR NOT REPLACEING.. .. on the trans tunnel and other area not going to be replaced ,, plug weld the ends to the inner rocker. . Then do this to the next floor brace if you need to replace..

Lasty replce the rusted floor OPNE SIDE OR ONE 1/4 at a TIME... The floor removed acts as a rough pattern tio the replacement pans and drill PLUG weld patern also..
Doing it this way you preserve the structral intregerity of the body as you replace the affected area..

I'm not Brian, But thanks for the info. Is it a must to jack the body off the frame?

Sorry about the name..
Yes IMO.. you must jack up the front at the fire wall area..
You have the room to weld the floor brace to the floor and the rocker with it separated ..
You don't have to remove it from the frame just remove the front clip leave the rear body mount to secure the body on the frame.
Then jack the body up in the fire wall area with a 4X4 cut to fit between the frame rails and your Floor jack,,
I used a 4X6 as a spacer between the body and the frame below the fire wall..