I was close, so close… Well ok, I had the correct manufacturer. Thanks to persistence of DeadinSide and with the help of Daniel & crew over at Hemmings, the mystery has been solved. 
It appears to be an early 30’s Cadillac 353. While I cannot be certain Mystery Motor came from a 1931 Lasalle the shape of the frame suggests that it could have. After a little more digging around I also found a 1930 Cadillac 353 Victoria Coupe that also fits the bill nicely. 
Thanks to everyone for helping to track this down and if you have any other weird/rare/bizarre motor photos drop us a line.
Mine engine mystery is solved!
By Jo Schmo
Oct 27, 2010