I bought a used hard cover from a 2005 GMC.
The front was the right width, but the fleet side box on my 59 flares towards the back. Since the cover has styling lines in the middle, I made two cuts to widen it instead of one in the centre.
Using a saws all I started at the back and cut towards the froth but left about 4" of material and then jet spread the joint and fibreglassed it. I didi the same to the other side.
Once it was all solid again, I cut the rear end off of the cover and took out a section to shorten it, and then glassed the rear back on.
I'd advise not shortening and widening it at the same time. Instead do it in two stages as described, otherwise you'l end up with a bunch of jigsaw pieces to put back together.
