8 1/4 gear question

Hey everyone, my name is Tyler I'm fifteen from upstate New York. I am restoring a '71 swinger and I just found a 8 1/4 suregrip to replace my 7 1/4 open. The guy doesn't know what gears are in it, but its from a '73 340 duster. Does anyone know what the standard gear was for the 340 package? My guess is that they are either 3.55 or 3.54 however the ratio worked out for a 8 1/4. Thanks in advance.