It's one thing to fix it, it's another to keep it that way.
The fix is fairly simple. Start with your boat on a flat, level surface - like a garage floor (put some carpet underneath the hulls). Rig your mainsheet on the "long axis" - from bow to opposite stern. "Sheet in" until the measurements are even, then re-install the trampoline. Keep the sheet rigged in case the boat works out of square while you're tightening the tramp. The boat will want to "spring back," so you might have to overshoot the alignment to make it square when you remove tension from the mainsheet.
The problem is that with a 20 year old boat (based on your sail #), once you go sailing, it's going to go out of alignment again - doesn't matter how tight you get the trampoline. The only real cure for this is to glue the boat together.
More important is to see how stiff the frame is. After you've installed the trampoline and tightened it as much as you can, lift one bow slowly while watching the other bow. If you can move one bow 6" or more before the other bow starts to lift, you've got a serious stiffness issue that needs to be addressed with glue (best) or shims (OK, but not as good).
In reality, the fact that the hulls aren't aligned doesn't slow you down as much as you might think. One blown tack will more than erase any gain from having hulls perfectly parallel. Most gains and losses on the race course are made in the "transitions" - tacks, mark roundings, jibes - and in reading the wind. Improve your technique and boat handling skills and you'll improve your finishing position!