replacing valve seals on a 318 procedure?

DQ, you do not have to remove the heads, but if you do not have an air compresor with the spark plug adapter to pressureize the cylinder to keep the valves up in the head whle depressing the springs to remove the locks, etc.....

A head removal will be needed. At the bottom of page 1 and somewhere on page 2 of the below thread, theres some info on what is needed to be done. A spring compresor tool of ethier the one shown or the bigger "Around the head" like unit will be needed to compress the spring.

The way it is done in the thread, the valve will drop and hit the table. A soft rag or rubber/plastic base will avoid any issues. If you use a rag to cusion the valve and hold it up on reinstalation, check under the valve for traped rag ends.
http://www.forabodiesonly.com/mopar/showthread.php?t=10508&page=1&pp=20