Is there different coils for different applications, or is it pretty much any coil for any engine??
I have a Chevy 250 straight 6 and am going to replace the coil. I am going to check out ebay but not quite sure if there is something specific that I need to look for.

Originally the L-6 used a unique coil from the V-8's, but functionally it makes no difference what 12-volt Delco coil you get. If absolute originality isn't a big deal, you can get a replacement at NAPA for about twelve bucks. graemlins/thumbsup.gif

a coil is a coil is a coil, only difference being, is that newer coils have the ballast resistor built into them. So you'll have to check to see if this is the case with what you purchase. if it does, then bypass the resistor on the firewall.

I had a guy from http://www.davessmallbodyheis.com convert my old breaker point ignition to HEI. He recommended an Accel coil (it's the chrome standard ignition coil). He explained that it would work fine with HEI and had the benefit of being oil filled to run cooler. The HEI coils, he said, are epoxy filled and easily overheat. I took his advice and he appears to be correct. I haven't had any issues with my Accel coil and HEI trigger setup. So I'd say HEI coils and standard breaker point coils are similar enough to work well. In short, the higher the winding ratio the higher the spark voltage. When you get into the thousands of volts, it would take a significantly different winding ratio to make much of a difference in spark voltage.