'Electra Glide in Blue'.
If you read Robert Blake's book 'Life of a Rascal' he describes how Guercio was useless and did not direct the film. The book is peppered with the kind of thinking and writing that you see throughout the film, so I believe him.
One funny story was that Steve McQueen called him up angry, having used Guercio for a new film they wanted to make because they'd seen Electra Glide and liked it----then McQueen was livid and confused because Guercio was useless. Blake told him to just make the film around Guercio, who to use and whatnot and they did.
If you look at his body of work regarding film it's quite believable since his body of work consists of only a few films, two involving Blake.