I think the size of your shop is a blessing and a curse. A portable AC unit would probably be ineffective, since most of them are in the 1-2 ton range, unless you are willing to spend some serious money. We actually permanently installed a few 'portable' units on one of the older classes of ship I was on to keep the control system cabinets happy in the gulf of Mexico. They were 2 ton units and worked well for that task, and produced gallons of water each day in the humid Gulf.
We purchased some of them used from a rental company. You may want to see if someone near you rents them so you can see how effective they would be before committing to the purchase. This will also allow you to see if you have the electrical load available for powering them.
I would start by recommending a setting up your fans to exhaust out a door on one end and draw in from the other. This might give you a decent draft. The old standard roof ventilator turbine could draw the hot air off and also induce a draft for free.
Swamp coolers work as well, and as you've mentioned above it is probably dry enough that a little extra humdity won't hurt.
If you want to go the AC route, maybe think of dividing your shop up into storage and working spaces, so you don't have to waste the energy trying to cool the entire thing. To cool a 40 x 80 workshop space in your neck of the woods you are going to need to be in the 5+ ton range depending on height of ceiling, insulation, etc. Breaking it down in size or insulating could cut the load required by half.
As with everything in life, the solution is limitless depending on what you want to spend!
I personally deal with the Virginia Humidity and heat in summer by adjusting my work times when possible, and I use a big fan to blow out of the shop. This keeps from stirring sawdust up all over the place. http://www.spot-coolers.com/about.php http://www.mobileair.com/sizing-port...r-conditioners