The offset bushing installed as directed produce more camber. Installed opposite of the directions can provide more caster. More caster is preferred over more camber because it will provide additional high speed stability, better return to center characteristics, and better keep the tire in contact with the road as the suspension cycles.
When doing an alignment, ride height is the first measurement set. I'd suggest you set this yourself and advise them to not change it. Then ask for as much positive caster as possible and balance that out with -1 to zero degrees of camber. The positive camber you have now was fine for skinny bias ply tires, but with the modern radials you have now, negative camber will help with road holding without odd wear.
If they are a computerized shop and can't input manual specs, ask for a 2005 Mustang GT spec. They won't be able to duplicate it exactly, but the closer they come, the better it will feel.