Don't know anything about the accuracy of this site,
http://www.mymopar.com/extcastnumb.htm
But according to the page above, it looks like you have a 318 manifold off a 1972-73 Dart (3614395) for the left side.
If you can't take the right side off, you might also try using some kid's modeling clay to get an imprint of the casting number on the opposite side if you can't see it with a mirror.