I run both dirt and asphalt, however I'm fortunate in that I have two cars, one is set up for dirt and the other for asphalt. I tried going the single car route but I could never really get the right set up for each switching back and forth. I typically race dirt on Friday night and asphalt on Saturday with the tracks about two hundred miles apart! So as having two cars makes a lot of sense for me in that it makes life much easier. I'm more experienced on dirt and this was my first year racing asphalt so I'm still learning.
As far as set ups, I race the same caster/camber, wedge, ride heights on both cars. The difference is in the spring wieghts, the dirt car has lighter springs, 200 LF, 225 RF, 165 LR and 145 RR versus 275/300RF, 250LF, 185 LR and 200 RR. This is due to the dirt track being rougher and sometimes deeply rutted and a heavier spring will not only beat you to death but will cause the car to bounce out of control. However some racers do run heavier springs than I and are very successful. The other difference is I run half the the tire pressures on the dirt car than I do for the asphalt car, and of course camber cut tires on the asphalt.
Your idea of having shocks/springs mated and ready to swap is a good one. Its a lot cheaper than two cars!
You may also want to have a set of uncut tires to swap for dirt also. I've seen cut tires run in the main on a dry slick dirt track, however they typically don't work well on a sloppy track during the heats.
I hope I've been somewhat helpful.
