1993 Dodge Dakota 2x - are all 3 different coil spring application's the same diameter - will a 4 cyl coil fit in a V8 a-frame and vis versa

The physical dimensions should be the same, but as you said, the spring rates will be different. If you are looking to lower it, drop spindles would be a better choice, because ride and handling would be the same as stock, just lower. Western Chassis spindles are the originals for this application, and are a high quality part for less than some other more well-known brands.