There are cross reference and replacement (for lack of a better term at the moment) but finding one of those, especially for the time period your looking for at the time is difficult. Few people saw the value of keeping these old books
Without the specific specifications for each you have little to compare except by trying the belt. By 1970 you saw the parts guys focusing more on the quick reference numbers, often starting with a "J" as you can see below. As you can see that the MPC from during the year the cars were being built didn't even list the engineering number but only the JB-345
Not sure where the -G1 came from. Possibly as a replacement or later in the production year but for discussion and reflection below is what Ford was suggesting and offering in Feb 1970 for the Boss 302 and for other applications as well as some other belts
The measurements IMO is the greatest help but I would be surprised if the belt you have is even close. If I'm reading the charts correctly it appears that its about an inch shorter. For our focus here and as long as you are using reproduction parts on your car I would go with a reproduction belt made for your application. But that is just me at this point in the discussion
