Spray foam would be best if it is within your budget and is available in your area. It serves much more purpose than just insulating as it seals drafts and acts as a moisture barrier as well. Dow Thermax or similar is also very good and requires no special tools or equipment. Staying with the 2x4 flat furring strips you can't get much more than an R 9.5 insulation value alone. The blocks have some R value themselves but it is negligible. If you add some more furring thickness you and get up to a full 2" you can get to an R-13 which is the equivalent of a fiberglass insulated 2x4 framed wall. If there is any way to access the void spaces then adding loose fill in the cavities will benefit you greatly. I hope this helps. Please let me know if I can be of further assistance. Thanks for using JA!