Roadnoise is the noise from outside coming in. Tire noise, other cars, wind type noise. I'm not sure how a baffle will help this. With a 6x8, you shouldn't need one. If you were mounting a smaller speaker or a round speaker you may need one. Basically, you can subsitute the word adapter for baffle. To cut road noise down, dynamat your doors, both the outside panel and the inside and try to seal any possible airleaks that you can. You will not be able to completely seal the speaker without some serious mods, though.
Dave