Solrac, if your still having trouble stretching your frame and getting everything to align correctly there is a very simple solution to this. Before cutting your frame you should use a telescoping rod for all four point that you will be cutting. you can still use this method in your current state and will work on uneven ground. I do this all the time and it takes out all the guessing when stretching, limos stretching is done this way, but with a better telescoping rod.
step 1. cut out the metal where you welded for the stretch.
step 2. move you back half of the frame back to its original connection points.
step 3. make your telescoping rods. you take DOM round tubing, 2" wide tube plus the length. your length is your inches of stretch plus 6" extra. Than get 8" of a second tube that goes on the outside of your 2" tube. Length of this one is 8". cut the 8" tube in half. Now make 4 more of these the same way.
step 4. Once your moved your frame back to stock dimensions, tack on the tubes to those points. Only tack your outer tube, do not weld where you have cut the 8" inches in half. After you have tacked on all your four points. make sure your inner tube can slide freely back and fourth, you lube it up if you like. When you happy about its movement, weld the outer tubes to your frame so that it will not move on you during the process. Must be strong welds as it will be supporting alot of weight during the stretch.
step 5. you will require 1 floor jack, 2 hand crank jacks that you find in the trunk of your car, measuring tape.
step 6. now you can start spreading your frame back with the hand jacks till you reach your desired length. use the floor jacks to lift up on the back corner of the frame if it seems uneven to you. measure your lengths, measure in the engine bay area corner to corner to make sure its square, than tack the outer tubes to your inner tubes so that it will not move on your when you go to weld your square tubing to the frame. The whole process should cost you no more than $50-60 in metal to do the stretch. You can reuse the telescoping rods again later on if you stretch another car.
Hope this helps, this is the way that I do my stretches and it comes out perfect every time. Takes me 30-40 mins to stretch a car this way.