In case anyone else gets to the lvl 70 dungeons, I used the below method:
1) Start saving lvl 60 luck marks asap. However, I only enchanted my gear to lvl 21 as it does take a lot of lvl 60 luck marks to make enough lvl 70 luck marks.
2) Keep leveling up until you hit lvl 76. At level 76 you can do the normal mode dungeon “Distressed Land” which gives lvl 70 luck marks. I used a few stamina potions that I had saved up and got a few hundred lvl 70 luck marks in just a few days.
All in all it wasn’t really too hard to get enough lvl 70 luck marks. Just have to plan ahead and save/spend accordingly.