I have a hard time spray basting poly. This might be a good time to try glue basting. Pins would drive me insane(r).
Don't pull your backing tight before you baste it. Just the same tautness as your front.
Painter's tape is a good idea or I usually just use regular white chalk lines drawn lightly on every 6-8 inches and sew depending on what the batting suggests. Be sure when you get near the edges that they aren't turned under on the backing.
I also would suggest a walking foot, if possible.
Start in the middle and work out to both sides
Let us know how it is going.
Watson