As Deuce Lover posted... This is exactly what I put in my 34... Pinion ground by a crankshaft grinder, to 33-34 specs, spacer added, driveshaft shortened. Luckily, my crankshaft guy was also a flathead guy, he knew what to do, and he took a 33-34 pinion, and measured it to determine the finished size for the later (37-38) 10-spline pinion. The spacer could be made, but in my case, I found an appropriate wheel bearing part with the correct dimension. (For my use, the 3.78 was a very good choice.)