Anybody know what length driveshaft I need?

This might help you identify what you have.
http://www.dennysdriveshaft.com/img/File/how_to_measure_diagrams/b.html

IIRC the 1310/1350 joint uses a tab outside the cap or snap rings to locate it. If the yoke has neither, it's not a 1310 joint.

7260/7290's use a snap ring on the cap to locate in the yokes.

The driveshaft you need is about 52" C to C (51.96") according to the service manual.