It COULD have been an 8 1/4"! They were phased in late '69. My '70 318 Charger had am 8 1/4" axle. They are also listed in the '69 Plymouth factory service manual.
If it is an 8 3/4" and is original, it may have the small 1 3/8" pinion '741 case. TRY to get gears or a SG for THAT oddball!