most frame manufacturers build cab supports into the trans support at the back of the trans
this is a roadster shop frame with big ol cab support bracing
whoa! big pic
total cost uses tubing braces to the cab supports
and this is how i braced my frame with the trans x-member for the cab supports on truk
1.5'' sq tube for the main x-member and 1'' tube braces to the cab supports
the center angle drops out for the rear trans mount