Hello
Are you using solid body mounts?? That apparently will help as well. A roll bar /cage would help, but I don't imagine you want to cut any holes in the body!
I have seen drag cars that have braces that run from the rear frame rails to the crossmember underneath the car, but I don't know how much room is under a stock vette at the rear. You may be able to weld in some diagonal links underneath in the main floor area, but the best way to strengthen it would be a cage
I know of a guy in Texas(Jim Moore) who runs 890 hp in his 67 convt. He uses a G-force 5 speed and has no cage at all. He goes 10.0's maybe 9's now, but only on some outlaw tracks . I think it was Vette magazine that did an article on his car a while back
take care
Tom