Martin / Swiss Mustangs
That's N851D(1) [44-84658] with N2051D [44-73454] behind - both Cavalier conversions seen here somewhen 1967 - 1969 Martin
Date is somewhere as early as 1961 and prior to August '67, when N851D got a Cavalier tail and a new paintjob. Airframe in the background _could_ be 44-73206 (N7724C) but the paint scheme isn't correct from my reference photos.
N851d looks like Tony Malhuttas mustang. Im fairly sure the mustang in the back is the mustang that Bob Hoover flys.