Do you aim to start the work soon after the Specification Review?

At the end of the Spec Review encourage the project to go ahead with the current resources, while it is fresh in their heads.

Say words to the effect of:

"The resources allocated to your project depend on availability. All efforts will be made to allocate the original resources used on the Spec Review, but that cannot be guaranteed.
If you decide today or tomorrow, then we can use the same resources...
Regardless, it is always better to move ahead while the project is fresh in the developers' heads."