More than you want to know about chip programming is over on Binder Planet. You can buy most everything from Moates, or make your own....depends on what you want to do. Go here
A tune from a 4.3 will work pretty well on the 4.2, but you may need to make some adjustments. In my case all the numbers from the run file looked good but I made a slight adjustment to the fuel pressure and things were pretty balanced after that. If it is really off, there are also any number of companies out there that will take a run file from you and then burn you a tune to correct what they see on the run file for about $100. If you want a custom tune, that is they give you one tune, then you run it, they tweak it and so on until you are satisfied with the results, there are several companies out there that will do that work for about $300 or so.