Still have an SM420 in my '62 C10. It was originally a 3 on the tree truck, but a PO cut up the floorboard and dropped the 4 speed in. I was able to find the correct floorboard sheet metal and column and it all looks factory now. I've had no trouble with it.
When I first got the truck in 2005, I took it to a local tranny shop and had them give it a fluid change and once over. They said it was in great shape.
From what I was told, these things are nearly bulletproof. They were put in everything from 1/2T to 2 1/2T trucks. My C10 has an L30 spec (275hp) 327 in it and the SM420 handles it with ease.
