having a difficult time identifying specification and then probably finding bolt to attach ground wire from battery to the front right(passenger side) of engine block.
The hardware required depends on year (I assume 1966), plant (it seems San Jose may have done things different), date of production (a running change), engine and transmission.
similar question on correct coarse threaded screw with hex top to attach ground wire to firewall.
This screw is standard, and is documented in the 1966 Mustang Electrical Assembly Manual (AM0013 pg 15 and 36 - same hardware, different engines).
The hardware spreadsheets in the library for 64 thru 66, have the required items listed by application - "WIRING GND,BATTERY-ENG,***(eng type) and "WIRING GND,ENGINE-FIREWALL@FIREWALL".
I recommend you get a set of the 1966 Mustang Assembly Manuals, plus the 1964-65 Chassis and 1965-65 Weld and sealant. They answer questions you haven't thought of yet.
Jim
Jim