Rear end help

[Leroy-B1]

So I bought a rear end from a local guy and he wasn't sure if it was a 68, 69, or a 70....The problem I have is I want to order the 5 lug conversion axle kit and not sure which one to get.. The brake drums are off and I am measuring from square flange on end of tube to the same on other side and the measurement I am getting is right around 56.25 inches from square flange to square flange...if I measure from backing plate lip to other backing plate lip its right at 55 inches...Can anyone help me? The kits are 63-69 and then 70 only...don't want to order the wrong one. Looked at old threads and didn't get a lot of helpful info that I understood...lol

Thanks in advance.