There's an old saying that comes to mind." Measure twice, cut once." Another good one is "Plan ahead"
You already made two cuts and you don't have any measurements to work with.
If you already have a pickup bed you just need a tape measure.
If you don't have a pickup bed get a 6 foot stepside bed. The fenders can be moved back or forward if you cut the frame in the wrong place.
You'll need to take another 4 inches out of the frame somewhere to shorten it 2 feet. Do the math.
All these details should have been considered before you made your first cut. Good luck.