The forward yoke of the universal joint is unique to the SM-420 transmission. The one that fits the 3 speed doesn't work with the 420 transmission, and I believe the crash box 4 speed U joint is also different. That part is very difficult to find, and expensive when you do locate one. It also incorporates the speedometer drive gear into the U joint assembly. The U joint cross and the rear yoke are the same as the ones used on the 3 speed torque tube setup.
Jerry

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