This one has a 1966 401M GMC V6. Flxible offered a refit engine mount kit. An owner could swing a complete air brake equipped GMC V6 Medium Duty truck engine very easily. WB did the conversion in 1971. Engine is in good shape mechanically but parts have kind of dried up and its pretty slow in the hills. Its was a major upgrade from the original Buick 320 straight 8 though. Did an Allison 1000 6 speed conversion in 2015. Can hold a gear via the shift pad and you don't lose speed on upshifts and downshifts. The Spicer non synchro 4 spd was kind of arduous and I needed parts for it that existed nowhere. The V6 has an SAE 3 round opening flywheel housing on it so this 2006 Allison with the SAE 3 bellhousing slid right on. Had a custom billet torque converter hub pilot built and the machinist modified a factory V6 flywheel and stamped Allison spacer. Stand alone electronics with a TCU programmed by Allison as a 5 speed(dont need the double overdrive), large Griffin cooler under bus ahead of axle, trans oil temp gauge, dash trans check engine light, custom mounts, custom trans and pinion yokes, engine relocated 6 inches aft and 1.5 inches to curbside to get it all in there and for driveshaft geometry. Tons of fun.