That's correct, no gland nut required.
You could make the length the same as the overall length of the pump body
and the nut combined, good idea I suppose.
From the Reeves book sizes for the 5/16" 'o' ring are;
Groove width - 0.095" (25 thou larger than ring dia! It allows the ring to
roll in the groove)
Groove depth - 0.060"
Make sure pump bore is well polished to save wearing o ring, and round off
all sharp corners on pump body entry and on ram groove.

Simon.