There should be a tag on the valve cover with the model numbers. But, it should be a Stringer 400 Hydro-mechanical shift. Would need the model number to be precise.
I agree with KaGee. Typically the 4 cylinders have the 400 stringer's. The V-6's and V-8's will have the larger 800. Not sure of the answer as far as how long this version used but is quite a long time, I know that!