if that one's too big, then get the next smaller one. they are pretty universal, and are sized the same as NPT plugs- so if you have a 3/8" NPT one (actually closer to 1/2" o.d.) that's too big, then get a 1/4" NPT (closer to 3/8" o.d.) one.
do you have a stock radiator, or is it a newer offshore replacement?