You might get some info regarding an adaptor from these folks.
http://www.egmcartech.com/store/Cadi...657605810.html
The other possibility is modifying a Hogshead to fit the Caddy LaSalle transmission using it as the adaptor along with reworking the input shaft. Its not likely that whichever way you go the use of the Caddy transmission will fit the OEM trans envelope on the 40 chassis which means reworking both the X member and the torque tube/driveshaft. FWIW there were a lot of flathead and OHV powered rods running around with a Ford toploader and LZ gears back in the day which is in keeping with your build idea. Unless you are planning to do some serious racing you will have significant $$$$ invested in the Cad LaSalle modification that might be better used elsewhere. JMO.