need more info!! what size furnace?
what you can do with what you have already is to cut holes with a hole saw in the duct until the static pressure drops in the duct to about .5 to .75 at that time you will be roughly right.
HOWEVER you probably will not get enough air flow across the heat exchanger with the 12" main duct and your furnace will cycle on the high limit.
If you are in the mid south based on your handle then you need to add more ceiling insulation and propably ok on sidewall insulation
In the north way short on insulation