I did mine a couple years ago. I bought the 2x6 cut them to length and wrapped with scrap carpet I got for free. I didn't through bolt because I was concerned if they ever became loose and I did t catch it they may scratch my hull. I brought all 4 bunks pre wrapped with carpet out the boat in the water and did the work in the parking lot. I clamped the new bunks in place piloted out the holes and put in stainless 5/16" x 1 1/4" lag bolts with lock washers. It's worked for me so far but I do check them every now and the. To make sure they are all tight.
Also I didn't use PT wood its never in the water long enough to matter.
Good luck it's not hard.