I think fiberglass is easy to work with. I don't know what the part that you are working with looks like but I would fill in the hole with a pre made fiber glass piece. Do that by building it up with a bunch of layers till it is as thick as the part that you are repairing. Do it on a piece of wax paper or a plastic sheet. When it dries it won't be stuck to it. Cut it to size when dried and fit it to the hole then put a fresh piece over all of it on the inside to attach it and then a thin piece on the out side to finish it. The outside piece can be made with scraps mixed onto the resin and brushed over the area kind of like a body putty. Sand it down as needed to finish. You can do the repair with metal but fiberglass will work out better I think with out seeing it. Be sure to sand down real good both sides of the part being repaired and clean it real good with MEK or the fiberglass will not stick and if it does it won't last very long. Clean your tools with MEK too.

The panels are reproduced.  My advice would be to just buy new ones.  The grain is going to be impossible to duplicate and have it blend in exactly.  The way those panels were made the mold had the grain on it.  The grain was imprinted in the gelcoat that was sprayed into the mold first.  Then the glass was laid over it. 

If you must use fiberglass originals I would talk to the person who says they can repair specifically about the grain, and if they guarantee it will blend perfectly take them up on it.  Or source good used panels (not likely I had similar decision to make with my '67 and just chose to replace all the interior panels in the back because I was tired of the beat up, broken, ill fitting originals). 

6sally6 wrote:

Is the interior color black?!
If so........make the repair (as above) and then spray both panels with Truck Bed Liner in the spray can. It will give a close resemblance to the camera finish you have now. Once it dries spray over it with a semi flat black to get a smooth to the touch finish.
I did the above on a console for my parts chaser truck. Of course I can see the repairs I made but the normal person won't notice it. By spraying both panels it will be even LESS noticeable.
OR.....drop the cash for two new ones that prolly won't fit right!
6sally6

BINGO!

I did as Sally did on my home made rear panels, and it's close enough that I'm happy, and no one notices a difference.

Bevel your edges is the Biggest thing to remember. You can then add matt to the back side of the repair. A trick so you dont mess up the front side it use aluminum tape on the opposite side. The aluminum tape is ideal for the repair because of how it removes from the surface. After you get some matt or the bulk added you can then cut some cloth so that it doesnt go past the beveled edge. Once the fiberglass is done you can use some primer to seal the repair. Then I usually use some high build primer and spot prime the area. While the primer is wet have a thing of steel wool handy. Blot the primer you will be amazed how it blends out the repair. It will look nice after blotting the next trick is to lightly sand the repair to flatten and feather edge the repair. After a scotch brite pad can be used to prep the panel for the fun part painting the panel. I used this method on a 1968 Shelby GT:500 KR convertible. Then end result was amazing. Now you could also prime the whole panel and completely retexture it as well...