The battery tray on my bird was a rusty mess - and the old style battery too painful to locate and buy - so I went shopping........and found that the least expensive source of these type parts is from the Mustang restoration trade. These parts below are about $20 total before shipping.Virginai Classic Mustang has:
(1) #EG262 - '67-'70 Base Plate w/ bracket
(2) #EG270 - "J" Clamp Bolt (need two)
(1) #EG267 - Battery Clamp
The base has a Mustang specific bracket spot welded under it - but it comes free in a few seconds of work. Then you must adapt it to the space allowed. When I bought mine, several of the Mustang outfits were selling these parts as a kit - as low as $15.00 - so look around a bit.