I think my ’35 with a Ross steering box is a similar arrangement, there is no provision for a cotter or grooves in the tapered spline to hold a toothed bend washer. I don’t know if original, but mine has a split ring lock washer.
I am in the same position, not sure how much to torque. The 1948 MoToRs manual just says to make sure the nut and lock washer are “tight’. Of course with the tapered spline it is very challenging to remove the arm with a puller after being heavily torqued. Split ring lock washers do not provide any resistance to loosening when flattened, the teeth can only dig in to resist loosening after the nut has turned enough for it to unflatten. This may be enough to keep the pitman engaged in the splines and start warning something is amiss from clunking and before disaster – maybe not.
My plan is to use a new grade 8 split ring washer so the teeth don’t blunt when the nut is reversed and use Locktite 242. Locktite is considered more reliable than split rings. I am thinking of doing around 70 ft lbs torque but in the end will do it more by feel.