There is never one definitive answer about whether particular agile techniques like scrum are appropriate in a given environment; the key is to tailor the methods to suit the environment, the client and the project.
For any methodology to work, you need to have the buy-in and support of the executive (i.e. the business interests), the users (those who will be making use of the products of the project) , and the suppliers (e.g. the developers). So the first step to deciding if an agile methodology is feasible is making sure that the important stakeholders understand what agile project management methodologies entail, and that they are willing to participate in it.
Many of the off-the-shelf tools from scrum won't work well with such a large team; for example, you don't want to have a stand-up meeting with 40 people every day (and given they aren't all in the same place, the timing probably wouldn't work anyway). For such a large project, you should aim to divide people up into smaller, more manageable teams (maybe 10 developers at most), and appoint separate scrum masters (or team leaders). For scrum, this would work the best if you can meaningfully divide the project up into a programme of projects, and give each team a different project (whether this is possible will depend on the exact nature of the project, and would be complicated if there are too many different functional roles). The Scrum of Scrums method can then be used - a representative from each scrum meets to share information between what the different scrums are doing.
There are reports of very big projects using tailored versions of Scrum with distributed teams and working out very well - for example see this paper, in which a team of 50 developers over three countries used a modified Scrum methodology to achieve higher productivity; they call their model Integrated Scrums, where you have multiple scrums, but the scrums are cross-functional and incorporate team members from multiple sites. All members in each scrum meet daily with the other members in the same scrum using tele/videoconferencing, and a representative from each scrum based at the central site meets in person with the other representatives to coordinate the entire project.
If you have difficulty getting the organisation to accept this type of methodology (it can't be forced on an unwilling organisation), I would suggest trying to focus on the most important aspects. According to this survey based study, the strongest factor in agile project success is the agile delivery strategy (i.e. delivering products early and getting organisational input, rather than trying a big bang deployment), then agile software development processes (e.g. look to eXtreme Programming), then team capability, and only then project management processes.
