You don't need to do the heads with the 551 cams. They are all torque down low. You will like them.

 

I think the lift is too much for your stock heads. Have your stock cylinders bored if you want to go with more displacement and definitely speak with one of the sponsors on this site. If it were my bike I'd go 98" with head work and the correct cam to fit your riding style. Worth every penny and not that expensive if you are doing the work yourself. FYI... I just installed a set of S&S 509 cams in my friends 2002 Heritage and he's thrilled with how much better it pulls when riding in the mountains.

 

You can't run the 551 with your valve springs. The 509 has almost the same specs but with lower lift.

That said, you're going to have the heads off and you're dealing with 14-15 year old parts. At the very least I'd get the valve seals replaced an the heads cleaned up. Might as well swap the springs to a set of higher lift ones while they're off anyways.

Might be worth posting in the engine forum, lots of experienced builders there that can give you more specific advice. I'm a mechanic but not an engine builder. I can put it all together but there are better people to help choose the parts.