There's the SM318/RH10 and the later SM326/RH12. Either one would work as a virtual bolt in, though the SM326 is a bit longer, and would require shortening the driveshaft. Shift linkage is the same. On the earlier versions, Truck and Passenger are identical other than shift arms, after '58, the Passenger version has a mounting pad on the taillhousing. Either version will work fine. Look to spend about $100-$200 for one.
BTW, I have an SM318/RH10 from a station wagon in mine, running 3.90 gears and 30" tires, I love it. I can run 75 mph at about 2700 rpm without any problems at all.
