Here's my input on this situation.
#1 - Cardboard is your friend.
Make one out of cardboard.
it does not have to be perfect.
This will give you the idea and master plan.
#2 - The side that will be seen the most is the left side.
So leave it alone.
I would shorten the (Front) right side cap to make it fit.
#3 - If you are working on this with the bed off the truck the front will be easier to sand and do the adjusting and fitting where you can see what is going on.
#4 - once you get a pattern in card board slice the top and bottom of the front side a sand it till it fits.
#5 - Fiberglass and resin and sand till it looks like it was Factory edition
.