Unsure of the spline count, but the boss is from the 70s-80s rwd Toyota (Celica, Crown, Cressida etc) family. If you choose a wheel with a deep dish, the spacer is not required - I only used the spacer as I already had the wheel, which has been on six previous cars I've owned. OMP, Momo etc have a couple of reasonably priced deep dish wheels in the 370-390mm dia range, the 350mm size is too small for a truck even with power steering. You definitely need a sturdy name-brand wheel, as it tends to be used as a grab handle when climbing up into the cab.