Just about everywhere I have searched, I cannot find a rear spring that gives more than 1.5" drop for a 66 Chevelle. I'm looking to drop my rear end about 3" to compliment the 2" drop spindles I have on the front. Does anyone know where I can find these springs, or if it's even possible to drop the rear that far without bags?

Contact Coil Spring Specialties, they'll make coils springs any way you want...

http://www.coilsprings.com/

I've gotten a quote from them, and they were a little pricey. I was hoping there might be an off the shelf option. Does anyone know why I wouldn't be able to use rear springs for a '67? They seem to be readily available in 3" drop, but for some reason they aren't listed for a '66.

I don't know what your budget is but the last pair I bought were about $150 shipped, which IMO is a pretty good deal for a custom made part. On top of that, when you place your order, you can explain what your goals are and they'll help you choose the best rate and ride height for your application. After you install the springs, if the ride height isn't where you want it, you can send the spring back and they'll adjust them until it is right.

The biggest problem with off the shelf springs is they're mass produced so the spring rate you think you're getting may or may not be right.

'66 springs mount to the axle differently than '67 springs.