The only brace i've seen - on an 1800 at least - is what's in this picture:

http://www.thepotters.info/albums/album13/DSC00670.jpg

Notice the box section at the suspension mount going under the floor? This is not an OE part of the frame. How much it increases the stiffness of the chassis, I don't know, but I do know that the floor section this piece goes under is already a very low part of the car, so adding this piece severely reduces ground clearance.

I've you're serious about increasing the stiffness of the chassis, contact John Parker at V-performance.com, but my seat-of-the-pants guess is the only effective way to increase the chassis stiffness of an old Volvo is to add a properly designed interior roll-bar or cage.
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(2) '73 1800 ES 'projects', g/f's '73 1800 ES