I've dealt with this situation and I agree with Pawel that that problem is lack of trust. In our case the lack of trust was due both to a failure to deliver in the past and to the client's personal need to feel in control. Despite our best efforts, there was a lot of tension between the development teams (2) and the client for the duration of the project. However, we maintained enough communication and respect that the same teams were awarded another contract from the same client. That was a mixed blessing, but that's another story! :-)
Has the client seen your list of 300+ activities? It might be reassuring just to know that you've broken things down into more detail than the 8 items in the current plan. It may also help to indicate whether each task is low (~1 day), medium (~2-4 days),high (~1-2 wks) or very high effort; again, that shows that you've thought a bit about the details, and it assures both the client and you that all of these activities can be accomplished in the time allotted. (Or it shows both of you that they can't, in which case the problem is different.)
Try offering the client options for how to proceed. All of the options should address the client's concerns in some way, and you should be frank about the benefits and disadvantages of each option. For example :
- We can make a plan that assigns each of those 300 activities to a specified week, and at our weekly meeting report on the ones that were finished. Advantage: a full, clear plan from the start. Disadvantage: you spend a lot of time making the plan, and a lot more time each week reorganizing the plan to account for unexpected issues that have arisen.
- We can make a plan that assigns tasks for the next 2-3 weeks, and leave the other tasks all listed, assigning a new week's worth of tasks as we finish each week. That way the client has a rolling 2-3 week window of plan, and you have more flexibility to organize as you go.
- Rolling window greater than 2-3 weeks but less that the whole duration of the project.
You may think of other kinds of options that are more appropriate to your situation and to the way your team works, and you may have to adjust as the project proceeds, depending on how long it is. No matter what your options and decision are, you need to make sure the client has regular reports of progress and a confident, clear explanation for why you fall behind, if you do.