Chassis stiffening--full frame car

[jaybee]

Searching and reading I see very extensive discussion of how to stiffen a unibody car, and it's very interesting stuff. What is the best approach if you're working with a full-frame car? I'd like to specify a budget that doesn't allow for full replacement frames like the amazing pieces from Art Morrison and no roll cage. My thoughts are to start with a welded X frame to brace the frame rails and a shock crossmember in the axle kick up area to prevent spreading. Thoughts on these and other modifications?