That 3.38 set of gears will make your current granny first gear of that SM 420 usable from a standing start.
That gear set will also allow you to you reach highway speeds without the engine screaming.
If it starts lugging in forth gear on hills, just downshift into 3rd.
I had a rear axle with 3.08 gears and a Saginaw 3 speed with a 235 in a '50 3100, and it had no torque in first gear, but highway speeds were very pleasant.
