If I were you I would not cut off the end of the frame, you see that last hole on the top of your frame? Once you mount your bed you will need to run a bolt through there to bolt on the bed, so try to keep the frame intact, just try to cut around the welded hitch "piece" with your grinder and a cutting disc, use a hammer to knock it outta there, and dont forget to wear face/eye protection while youre at it. Just my opinion.
