[KTL]

For the axles to work in the '73, you need to change to the transmission flanges like you said. The transmission flanges you will need are not just bolt size dependent (M8 vs M10) but are also flange spline dependent. Some flanges have axle splines that are "fine" and some are "coarse" I do not know which years are fine and which are coarse. I'm just alerting you to the fact that there are different spline counts and you therefore must make sure the replacement flanges you get do indeed have the right type of spline to match the receiving differential housing inside your transmission.