It is better to put the ducts inside the house if you can.

if primarily a cooling climate, I'd go mini splits and can the ducts entirely.

Here is a link that might be useful: See this article and others at Building Science

Google the furnace make and model and see if it can be converted to horizontal operation. Attic installation of ductwork isn't ideal but it works - over 50% of all houses in the US have it!

This post was edited by sktn77a on Thu, Mar 27, 14 at 22:16

We've already checked on placing the furnace in a horizontal position and it isn't possible.
I was concerned with whether the fan would be able to blow enough air with the angles it has to go around. It seems like there would need to be more force to turn 180% and blow upwards and then turn 90% to go through the attic.
Am I worrying too much?