What the hell? Not sure why you want to get rid of a great tranny (that you already have).
I know that the 420's and 465's are bullet proof and cheap, I have owned and driven a ton of them. The biggest problem I have with them is the gear split and the lack of overdrive. The jump from granny low (6.54 or 7.02) to second (or L to 1st depending on year) is huge. The second gear is horribly high, something like 2.99:1, consider a modern truck 5speed has a first gear of 4ish or more to one. The six speed in my truck has a 5.71 to one Low with a 4ish to one first gear. SM420/SM465's are more like a crappy three speed with a compound low. I don't really like them all that much when it comes right down to it.
Personally I like the stick and if money wasn't a issue Jack is right a NV 4500 would be sweet. Have you thought about streching the 40, with the flat side panels it is fairly easy and you would like the extra space. The one thing you are going to not like as compaired to the 62 is the wheelbase on steep climbs.
