Thanks for the responses..

Are there differences in the IV, V, distributor? Differences between EFI and Carb motors?

If your 502 has a roller cam you may need to change the distributor gear to work with that cam.

As far a the differeces you ask about, Sorry, I don't know. But some one here should be able to answer that for you.

Sha Mat,

Save your self a lot of agravation! OK, there is nothing wrong with the Merc setup, but for $315 you could have a new DUI complete ignition system distributor and all! There are only two wires to hook up, igniton(purple) and the tach (gray). I have been running mine for three years now with no problems. You just tell them what you want for advance curve, and you have it at your house in a few days! Just go to: www.performancedistributors.com
If you have an HP500 cam, I would suggest 20 degrees in the distributor with it all in by 3500rpm. You really should find out exactly what you have for a cam to set the ignition properly.
Make sure you know which gear to run with your cam!

z.zuperboat, Your pm box if full. I will e-mail you later.
Thanks,
Joey