The SM 420 was discontinued in 67. If it is a 68-69 it will be a SM465. It is the same procedure for replaceing the rear seal. Like said previously, you do not have to remove the rear cover. To get to the front seal remove the front brg. retainer. Nothing will fall out but don't let the shaft & brg slide forward more then about 1/2 in. That will be a good time to check the front brg. Spray A little brake clean on the brg. to clean it & look in with a light as you slowly turn the shaft. If it has any pits on the balls or races it needs a new brg. It is common for this brg to go bad & it will cause whinning noises in the lower gears.
George
It's an SM420, I verified that years ago. I'm thinking I'll install it after replacing the front seal and checking the bearing. That way I can lock it in gear and won't need to make a yoke holding tool for a one-time job. Last night I found the shifter and put in the trans, and shifted through all the gears with it on the stand under the eave of the house. It has been a long time since I shifted that thing, it brought back some memories, really looking forward to having it in the truck.
