[400 4spd.]

For that kind of money I would want nothing less than PHS documentation, and go from there. Also be sure the VIN is for a 66 GTO convertible. And lastly, (no offense meant by this,) if you aren't sure what differences there are between body components, do some more homework on GTOs before jumping on this one. Too many have been cloned from Lemans bodies.
Quick list;
66 and 67 fenders are different at the point where the header panel meets.
66 GTO and Lemans used different lower body trim, hood, grille, turn signals, and taillights. All of these are bolt ons. The rear body panel (tail light panel) is welded in. GTO was different in that the tail lights are much larger. If the welds around the inside (where it joins the 1/4 panels and floor) look like factory spot welds, that's what you want. To use GTO lights in a Lemans, the rear panel has to be replaced. Also see if any emblem holes were filled in around the GTO script on the inside of the 1/4's. For instance, where "Lemans" used to be.
Just my opinion...