The reason the head is different between the two engines, I'm sure is combustion chamber volume. When displacement changes, the combustion chamber volume has to change to keep the compression ratio correct.

I've never even checked to see if the difference in these two engines is bore, stroke or both. But if you insist on going to a 3.0 displacement engine, your best bet will be to buy a long block. The other item you must consider is that the injection system is set up for a 2.6 engine, not a 3.0. You must research the injection system thoroughly, it may be that you only need a different CIS unit, or it may be that all electronics must change. I expect that the throttle body is a little smaller on the 2.6. If so, you would have a restricted intake system and would gain little by increasing the displacement.

If you really want performance, on this 3.0, and are willing to give up mid range and smooth idle, use an early '86 camshaft.

You will need to do lots and lots of research before undertaking this project or you will end up with a dog.

Of course, the easiest way to get more performance is to buy a different car, it will prevent many disappointments and headaches, guaranteed. With that said, if you have the space, equipment, tools and total, committed desire to do this project, in that case it will probably be therapy.

Good luck,