The old Chrysler Racing manual had an untouched head pecking order, when you could not port for Stock and Super Stock Class racing. Prine is pretty much right in general, except the 70-71 340 J heads had 2.02 intake valves. The best heads were X heads, then J / O / U, then the next in line etc. They got progressively worse till the 308 castings. However once you start porting there is not much difference, not that I'm a porter, I just clean up the bowls, gasket match the ports but leave the bottons untouched, equalize the chambers within .2 cc and go. Early heads rarely crack, smog heads should always be magged. I've given up on finding uncracked 308's and I'll never run aluminum heads. X's, J's, and 587's are fine with me. The 587's and up had hard exhaust seats. The general pecking order is the same for the small port 273 / 318 heads. The older the better. Personally 340 engines were pretty much perfect, HP 273's next. 360's and 318's last although the 74 HP 360 was pretty good. They all have pluse's and minus's TQ carbs and manifolds are the best street induction, the trick is to find a good one now. I guess the good ones are all about 40 years old. I still like the 340 cam, but I drop the compression from the 68 to 71 340's. All Chrysler small blocks can be made to run very well. Some have all the good parts to begin with.
