Since you've already spray-basted to the backing, I'm not sure this will help.
You can separate the batting into 3 pieces. This eliminates the bulk on each side while you are machine quilting the center. I first saw this technique described in detail in a book by Debra Wagner. Marti Michell now has a book out on how to do it. A long time ago I posted how to do it, and someone later PM'd me that it had worked very well for you. Later on, if I can find my old post, I will link it here.
Haven't found my old post (200 pages of old posts to work through!). However, here is a link to the Marti Michell book:
http://www.amazon.com/Machine-Quilti.../dp/B000NNGZJE
I'm not absolutely certain, but I think this is the Debra Wagner book that has excellent instructions on this technique:
http://www.amazon.com/Traditional-Qu...dp/0801986605/