The yard that I called said that I would have to look at a couple of theirs that they had to confirm everything. It is here in Albuquerque. I am also talking to some hot rod people that might be able to get one from a 1950s era Chevy truck. If I find a couple I will put up a post.
I paid $40 for my 1964 SM420. Keep looking. You can rebuild it yourself, it is pretty basic. $200 wold be ok it it was rebuilt though if you don't want to rebuild it yourself. But $200 is a lot for one that needs a rebuild.
I wouldn't pay over $50 for one that wasn't completely shinny and brand new inside. There are a million of them around you just need to look. Start going to swap-meets. You'd be surprised what you can find there.
I got miRe: how much did you pay for your SM420?
Don't forget to look in the big GM 30 - 50 series busses, 2 ton trucks and delivery vans. Some of these have less mileage than pickup trucks of the same vintage and you don't have to jack them up to get underneath and drop the tranny. I got mine out of a 62 bread delivery truck for $50.
If you (or anyone) would like to help me with what seems to be the hardest part (pun) of this project then I can compesate with a finders fee. Being tied up 8 to 5 M-F makes junk yard searching a real pain. The next few weekends are trouble for me so I would be willing to compensate anyone with a "finders fee."