3dTstat has some options that may be useful for you:
-max = compute maximum of input voxels [undetrended]
-absmax = compute absolute maximum of input voxels [undetrended]
-argmin = index of minimum of input voxels [undetrended]
-argmax = index of maximum of input voxels [undetrended]
-argabsmax = index of absolute maximum of input voxels [undetrended]
Are any of these what you had in mind?