X-style crossmember?

Convertibles don't have a roof and most 60's/70's convertible were unibody and didn't have a frame. GM midsize and Full size converts had a frame, but I don't remember an X crossmenber in my Chevelle Convert.

To stiffen up a G-body the easier thing to do is install all of the lower frame mounts. Most G-bodies have missing lower bushings from the factory, for a cushier ride. Then an X-brace in the trunk and jounce and fender bars in front. Use the body (especially the roof) to stiffen thing up. If you have T-tops, install a roll cage or buy a hard top.

what cars used an X brace on the frame? The only cars I am aware of are Checkers and vintage Fords had an X transmission support and GM used to have an X frame...

I found some more info but it was the frame that had the X, not the transmission support. In 61 they went to the perimeter frame....

GM convertible cars of the muscle car era had boxed frames. A Chevelle or A-body with a boxed frame had a narrower transmission crossmember. Camaro and Firebird had extra braces and dual floor pans in some areas. Fox body Mustang convertibles had a dual floor plan too IIRC (as did police cars)?

In GM cars the X frame on Impala's was 58-64 with a carrier bearing in the middle. 1965 Impala had a normal style frame, as we know it.

As for a G-body, I think it has to do with packaging. They didn't incorporate dual exhaust from the factory. Having exhaust ran around or through that would seem to be a nightmare, unless it was really small.

For reference, you might look at some 25.2 and 25.5 chassis in race cars, they sort of add another inner perimeter and lateral braces.

The X-frames are awful in a offset crash. Add so rocker rust and a 20mph offset crash is deadly (even with a seatbelt, which they generally didn't have).