Re: 1956 johnson, need parts
Look for a complete used lower unit. It will probably be cheaper than buying the housing and prop shaft and you'll have a spare gear set to boot. That lower unit was used on the 10 and 18 hp from 1956-1959. I think there was a change in 1960, but it may be possible to use a newer LU from the 1960's. I don't have my books in front of me right now, but I'm sure someone else will come along.
Check eBay, post an ad on craigslist, or post a free ad over at AOMCI.