Switch it out! A good carry handle mount (there arent many) will cost you the same as a new flat top receiver. You will not have use of your iron sights, and you are looking at about a 4-5 inch hold over. Also, you will be putting your chin or jaw on the buttstock instead of your cheek. If you are still determined to mount a scope up there, find an old Colt brand scope or one of the knock offs. They have the mount built into them which helps lower them a little.
This will be the scope mount you want to keep it as low as possible, made by DNZhttp://www.bing.com/images/search?q=ar15...selectedIndex=0
This is a variant of the Colt brand scope, they can be had in a 3-9 powerhttp://www.bing.com/images/search?q=ar15...selectedIndex=3