Chris-Reilly, most of the post came from an article written some years ago by Peter Brock himself, supplemented with photos and their comments from Phillipe Olszyck's 2006 book on racing DeTomaso cars. Peter also gave a rather informative talk on the subject during one year's POCA Fun Rally when he was the main speaker. He's about the only major player still around today, and a nice person to talk to.
As far as Shelby's side of the story, when he was still around he mentioned in an interview that Ford had ordered him to pull the sports-racer money and use it on the Cobras, which were far closer to success than the unapproved new sports racer series. So he reportedly said, 'OK- I'll take the heat. Hope things work out' and chopped DeTomaso off. As mentioned, the Cobras- thanks to Brock's Daytona coupes- won the '65 WSCC championship to help rescue Ford's loss that year with the GT-40s.
As it turned out, the FIA class in which the sports racers would have competed was in the process of being cancelled; maybe Ford at their level were already aware of this and saved a mil or two. But we enthusiasts did get the Mangusta in return and I suspect DeTomaso turned a profit on the whole deal in the end.