You are looking for ways to bring a project plan in on budget and on...

Use students and volunteer labor. Since these individuals work without payment, they will extend your workforce and accomplish the work under close supervision of skilled staff.

Plan to have 24x7 shifts. This will decrease the time length of the project, which will cut the staffing requirements.

Ask contractors to cut their staffing model to match your budget. Contractors tend to over-budget staff as a way to build in contingency.

Use in-house staff that either have the desired skills or are seeking to develop skills and experiences. This may mean negotiating trade-offs with other projects or departments.