What you actually need is a body brace which goes across the transmission tunnel. I did one for mine many years ago, photos here
http://www.suzuki-cappuccino.com/albumv ... dev=1&a=31There was a short thread on doing it here
viewtopic.php?f=2&t=631&p=3050&hilit=body+brace#p3050The essence is to stop the two sides of the tunnel moving fore/aft relative to each other as the body twists. I did it with a cross-braced welded fabrication picking up at the "hard-points" of the front and rear subframe bolts plus using the fixings for the heat shielding for extra clamping, but a simple flat plate would equally work. Some production convertibles use an "X" frame which fits between the front/rear subframe mounts.
The main task is to make sure the exhaust doesn't foul it and to maintain adequate ground clearance.
I never did make any sort of drawing, I did it as a one-off measuring as I went. As commented in the thread, the issue will always be variations in different bodies and general tolerances. The plates on the subframe fixings were 3mm thick, and I used the standard bolts on the principle that there was still enough thread engagement.