We've had great success with our direct-fit rear clip:
Available in triangulated 4-bar and 3-link. The tri 4-bar clip requires no floor cutting whatsoever, and the three-link allows you to still retain the rear seat. Imagine being able to replace your .045" thick 2x3 frame rails with a mandrel-bend 2x4x0.120 welded assembly. Talk about a stiffness improvement! You can keep your stock fuel tank too with minitubs.
Same suspension used in our 3G Vette that turned over 1.05G on undersized tires. It works!