not sure if the 40 is the same width as a 47 Desoto but I put a 68 c-body 8.75 in my Desoto. it dropped in with the stock perch locations. the rears in these cars are wide, I tried(because I had them) both 65 & 69 body 8.75 rears and they were to narrow. I did have a 9.25 rear from a 79 ramcharger(any truck would work) in the car for a while and it fit too.