is it possible/hard to do - 5 to 6 lug pattern?

[Pyrotechnic]

Keep back spacing in mind. I've seen people install 6 lug wheels off 99+ trucks with adapters, and the spacing ends up just right. This is because newer vehicles use much more backspacing than older ones.

If you convert your truck to 6 lug, there's a good chance the wheels will be really sucked in too far, and you will need spacers anyway. Adapters work out well because they fix the bolt pattern and spacing issues all in one shot.