I have done this conversion on one of my trucks and it's not really that hard but might be a little expensive depending on your access to a machine shop and driveline shop. Once you enlarge the hole in the bell housing the 465 will bolt in then the drive shaft will need to be shorten just a bit or I did on my truck. When you get finished underneath you will have to modify the bolt in portion of the floorboard. The 465 is taller and the shifter is a little father back. The SM420 is a good trans. the only reason I switched was I had a 465 on hand and didn't want to rebuild the 420. Hope this helps.