Using Cussion's information, I located the Grateful Dead concert that includes Pete and Duane: February 11, 1970
. The link takes you to the Internet Archive of the sound recordings of the concert in MP3 and other formats. (Over 3,000 other Grateful Dead concerts are archived there. I'm a relatively new convert to the Grateful Dead. Any GD fans know of a concise or searchable repository of guest musicians at GD concerts? Without Cussion's information, it would have just been blind luck to stumble on a concert that had Peter Green.)
From the recording notes
COMMENTS Late Show. Total time 1:26:37 +. Duane Allman's guitar is audible from near the beginning of Dark Star through the end of the set; Gregg Allman adds organ & vocals during Lovelight, and Berry Oakley plays a second bass on Lovelight. According to Kenny Schachat, who attended the show, the guitarist audible starting about 2:30 into Dark Star and for several minutes thereafter is Peter Green, not Duane, who enters around the middle of Dark Star.
"Spanish Jam" (follows "Dark Star") sounds like live Fleetwood Mac, although Pete's guitar (I assume) is faint. Perhaps someone with a better ear for guitar players can listen to "Dark Star" on and tell us when to listen for Peter's playing.