This is currently on ebay. I've heard that a few Cadillacs and Buicks were converted in the late 50's. Judging by the VIN, this is a Limited Chassis. If you look at 1958 production data, there were about 30 or so Limiteds listed as "chassis only" in the production numbers. I wonder if this is one of them?
http://cgi.ebay.com/ebaymotors/Buick-1958-BUICK-2-DOOR-MOTOR-HOME_W0QQcmdZViewItemQQcategoryZ6144QQihZ018QQitemZ280225143211QQrdZ1QQsspagenameZWDVW