Make sure woodruff keys are seated in pinion shaft. Dry fit gears on table to check for possible dings and burrs. (clean up as needed)
Need to make a liner tool. Gears and spring set up on liner tool to make sure it's clocked right. A little welding with a pieces of key stock and bar stock will work. Service manual may give more detail
Squeeze gears and spring together and then use a couple pieces of wire to keep them compressed.
Set the squeezed and wired gear assembly (still on liner tool into tranny case. Bring pinion shaft in from rear. Make sure woodruffs nearly level or have a slight tilt to front. The pinion shaft will make contact with liner tool and will push it out the front as you go.