The drivers side middle bottom bolt is difficult to get at.When I did mine I took that center bolt and put it through the header first and put a strip of tape on it to secure it.
Then I lowered the header down into position and very loosely hung it with the two upper outer bolts.
Then I went back under the truck and pulled the tape off gently and started the center bolt.You just have to find that right position so you can get to it.
Then it's back up top to start the rest of them.I used a wrench to tighten the bottom center bolt 1/4 turn at a time
You should have had all your bolts started before You even thought about tightening the any of them.
Hope this helps and good luck.