So Bill now you want to build a GTO?
I have it on good authority that Gilco Autotelai in Modena made all the frames from that period. Ferrari, Maserati, Stanguellini, Alfa specials. etc.. And your in luck they are still in business
http://gilcodesign.com/it/ I think they mostly make custom tubing today but they provide information for restorers.
Good luck.
Call me sometime.
Tim