The S13 rear section can be described with many words, but "strong" and "stiff" are not one of them; this is caused by the hatch design. The upper rear is full of emptiness.
A pic is worth a thousand words:
Note there is a "missing" part of the frame rail. Some chassis have them, some do not.
If rules allow you to cut a lot of things and use tubes as subframe mounts, you may want to go that way.