[Gromit]

Since no one else has yet replied, I guess you're stuck with my admittedly inexpert opinion if you are looking for something to cogitate on for the moment until the big dogs weigh in.

I believe that you need eight bolts in your cross member: one each front and back and one each on the upper and lower of each side. Just like General Motors designed in as original or stock; and the steel needs to fit tight between the chassis C-beams.

I really don't get these aftermarket cross members that attach with two bolts on each side or only on one ear of the main chassis c-channel.

So in other words I am suggesting as a first run approximation ruling out any cross member that only attaches to or bolts to one of the channels of the c-shaped main chassis beam. Whether or not it is made of tubing or bent flat steel.

Hopefully that makes sense; I just had three pints of Coors Banquet.