Good idea on the plates, well done there, I would definitely be going for a larger high tensile bolt and use shims to change castor, also use a larger rod end. Can you buy from ebay US? A lot of surplus race gear on there.

As for measurements, monroe catalogues here in Australia have all measurements for their shocks, good for sizing up different models and makes doesn't state valving, I wouldn't use gas inserts in a datsun myself. Oil is the way to go.