Does anyone know what the overall length of the positive battery cable on a1970 Cutlass supreme? And does it need the small accessory wire too?

 

The length should be in the auto parts store application listing.
One question is do you have a top mount or a side mount battery?

https://www.napaonline.com/en/search...ble&referer=v2

And yes, you need the smaller wire to connect to the power distribution stud on the horn relay.

 

Nov 1973 parts book says 53" for all A-body V8. OE part # 8907374.

 

Length may differ with auto vs 4 speed if using factory routing.