Just wanted to say congrats on your find!
I will also chime in and tell you that I have a set of original molds (DGP made) here that came from IRM after Jim Sutter bought them from DGP.
Point I wanted to make is, that when I got the molds for the fastback, it was intended to be bolted together to make a one piece rear clip like the one you show in your pics.
All the other versions, the notchback (two different models) were already cut up to produce separate rear quarters that had to be grafted onto the existing sub-structure after you "peeled off" the factory skin where needed.
So to help with shipping costs, I sectioned the mold to be the same as the notchback versions. It could still be reversed and made to be as it was originally, but like I said, it was done to help with shipping, as shipping that one piece rear clip would be fairly expensive to say the least. I believe DGP and IRM both produced the one piece rear clip. Not sure who sectioned the notchback molds, but had to be one of those two outfits.
Not disagreeing with John, as I know he knows his stuff! But there is also a set of molds that came with all of the IRM stuff that Doug Kinney got when he bought out IRM, lock, stock and barrel. It is for the race body and is intended to be mounted on a custom built tube chassis.
So I can't tell from the pics, which version you have, but it "looks" like the street version that mounts directly to the factory space frame. If it's not, you will have to do some major mods to get it to mount to a factory space frame.
I've never made a body from the race body molds, so really don't know how it's set up. I have made several bodies from the street version, which like I said, was originally set up to produce a one piece rear clip. The fenders look to me to be from the street version kit, but again, I really can't tell from just the pics. Do you know what the previous owner had the body mounted on? If it IS the race version, maybe somebody else already did the hard part for it to easily mount to a factory chassis?
At any rate, looking forward to your build, and do post updates and pics as you are able. If it goes like any of my projects.....it may NEVER get done!LOL