From INCO's website you'll get their phone number. Give a call to them, ask to speak with the technical dept. and make the question directly to the technical people. I know INCO from my college days forty years ago and they have a lot of excellent publications covering all aspects of the alloys they manufacture.
Another source of information is Huntington Alloys, from Huntington, WVa, an INCO's subsidiary for high technology applications of INCO's alloys. I don't know their website nor their address or phone number, but it shouldn't be difficult to find them out.
Good luck
Giovanni S. Crisi
Sao Paulo - Brazil

Inco Alloys have been very helpfull in the past when I have had questions on welding any of their products and even when we have been welding someone else's.
A person that I have talked to in the past is Sam Kaiser. He is in their technical department and can be reached at 1-800-624-3411 or 704-465-0352.
You should have no major problems welding Monel 400. It should be treated as any other high nickel alloy.