You can't gain alot of height in the rear because of the spindle angle. If you lift it a bunch it will have no caster, The steering will be uncontrollable. On the one here we upgraded to the '78 and newer rear suspension. It isn't much of an improvement but, is just a little better. If you want lots of suspension you will have to go to a newer chassis.
The best part about the '77 is the fact that it is light. Ours is real close to 300 lbs. I have a a '73 el tigre' as well, Me and my skinny buddy can pick it up and slide it into the back of the truck with ease.
I plan on building an open mod based on a '77 El tigre chassis. Drag sleds don't need much suspension. I'll post it on S.W. when I start on it.
The TXL is upgradable. you can put all kinds of suspension travel into it. Sinse the '77 is missing the motor already maybe you should thing of doing what I am, build a drag sled.