Hi, Dane-
Quick question---have you processed this data using afni_proc.py (for example, did that form the basis of your initial 3dDeconvolve command)? The benefit of that is that there are useful QC images for looking at seedbased correlation for resting state FMRI (and for task-FMRI, very useful images of the full Fstat and various stimuli/GLT effect estimates and statistics).
I am not familiar with 3dSynthesize. I am not sure I would start with an investigation of correlation mapping using that.
--pt