Shorter pushrods will definitely account for the rocker bottoming out. Look at it this way. If you have to run the nut down farther to get the rocker arm down to the end of the too-short pushrod, then you'll be partially opening the valves. This will result in severe loss of power due to lack of compression building in the cylinders. It will also result in the spring becoming over-compressed and experiencing coil-bind, which is probably the horrible clatter you're hearing.
Solution? Well, if you want to check this theory, go rent a compression tester and check the compression. If you're not building compression, it's a good sign that your valves are hanging open and not closing all the way.
If everything is stock and neither heads nor block have been shaved or decked, then measure the length of the pushrods you have and call Summit Racing (800-230-3030) and ask them what the length is on stock pushrods. If your rods are shorter, you might have just found your problem.
New pushrods are not that expensive. Just be careful to get the hardenned pushrods if you have guide plates in your heads. By trying to cut corners like this, you could end up destroying a much more expensive set of valve springs instead, or maybe even valves.
So before you run it one more time, I think you need to go after that compression test...or at least pull a pushrod and compare the measurements with a standard stock pushrod to make sure you at least have the right ones...since that is an unknown right now.
