the SM-420 4spd manual has been a tried and true design seem to do rather well in my truck i have towed loads as heavy 23K-LB's. . . tho my old GMC V-6 engine only makes [email protected] and [email protected] . .I do believe they also put this trans behind some of the stronger GMC V-6's. . .from 60-74. . .Ido believe this same trans went on to be used with 427's n 454's. . .