Garcia's appearance with Traffic was originally thought to have occurred on 3/18, but is now confirmed, via Arbunich 1968 (brought to light by commenter Steven Johnson at LLD), to have been 3/24. Arbunich's account supplies the following details: Sons of Champlin played first, then Traffic (the trio) did "Heaven Is In Your Mind", maybe some other tunes, and "Dear Mr. Fantasy". Garcia came onstage, plugged into Winwood's guitar amp, Stevie jumped to the organ, and a jam session ensued with up to eight players onstage at one time.
expost: Arbunich, Martin J. 1968. Strike, The Park, And Other Things. Guardsman (City College of San Francisco) 66, 6 (April 3, 1968), p. 3, via URL The Guardsman.