I don't have HEI, but do have an MSD5 box. Made an incredible difference in idle smoothness, and now need much less choke when cold.
I am running AC R45S plugs, and think this is the best for street use. Much less prone to fouling than the R43S. I don't think adding a hotter spark means you should go to a cooler plug. In fact, in my opinion the best thing to run on the street is the hottest plug that doesn't induce pinging. If it doesn't ping with ideal timing curve, then a cooler plug is just going to set you up for more frequent fouling.
