You can use 7940 for IPMA with CCM 334.
The Manager Phone needs two lines one for Manager Extension & another for Intercom. Assistant Phone needs atleast two lines one for Assistant Extension & another for Manager Proxy Line. You don't have to have Intercom line on assistant phone. So you should be able to use two 7940s to get it working.
If you need the same assistant to be assistant for more than one manager & need intercom, then you need 7960 or 7960 with sidecard depending on the number lines for the assistant. For manager's Phones you can get it working with 7940, you won't have any rollover line though for the managers since one line is used for the extension and the other for the intercom.
Short Answer: You can use 7940.