@kacomess - somebody else who knows 'Pearls Before Swine'! I don't think I'd call them prog as much as Tom Rapp seemed more like the more surrealistic Dylan stuff. Their first album, 'One Nation Underground', is also excellent...
I'm a big fan of Caravan and the whole Canterbury scene as well... Actually got to see them in Berkeley in the late 70's....
Zappa's early stuff on Verve is by far my favorite of his...