Does the trans start in 1st, shift to 2nd but not 3rd ( or higher ) ? Or does it start in 2nd and not shift into 3rd ( or higher )?

I made an adapter for a 722.5 ( the 5 speed version of the .3 / .4 ) from a 6 mm thread bolt drilled through the center and a -4 JIC hydraulic adapter or male plug welded to the head.

When selecting a gauge, ideally you want your measurement range to be 25 to 75 % of the gauges full scale reading. You can use the low end of the scale with some loss of accuracy but don't go over the full scale reading unless the gauge is retarded.

Dry gauges are just fine for what you are doing.