To be honest, I would still do the MPI again. I get better mileage now and she just runs so much better. The one downside is that I have to use a minimum 89 octane gas. Be careful when you remove that hamonica balancer, I ended up crushing my pinky between it and the radiator. Live and learn. Good luck!
From everything I've read about the conversion, the 4.0 head adds an additional amount of horsepower, 25-40hp, and I believe cures the pinging problem. I haven't heard of someone having the pinging once they've put the 4.0 head on. According to Tim Weaver's write up the '81 to '87 year 258's do need to be tapped for 1/2" bolts to replace the 7/16" bolts. But I surely don't want to give out any bad information, this is what I have gathered and believe to be true through my research about my own pinging. My next step is to put the 4.0 head on. Maybe someone on the board can give a definative answer and let us know if they have the 4.0HO head and still have the pinging problem, and if they can run lower octane gas.
You can use the 7/16 bolts by using alignment spacers to bolt the head down. HESCO and I think Summit or Jegs have the spacers available. Drilling and tapping for 1/2 in bolts is still the preffered method though. I was lucky, my '80 block used 1/2 in. I think you should seriously consider the 4.0 head while you're at it.