The OP's very first post, and this is what you guys throw at him?
That's a joke, nobody get ruffled. I respect the physics, math and other science applied here.
Having moved untold loads of lumber myself, my opinion is:
For the load you described, either way is fine, so I wouldn't really over think it. Some variables to consider are:
- Your load/unload preference or conditions.
- How far you're traveling, and the road conditions for your trip (rough, all smooth, stop/start).
- Type and condition of the 2x6's. Regular fir/pine/spruce framing lumber? Pressure treated? Wet/frozen?
- Pressure treated and/or wet/frozen can increase weight significantly
- Frozen boards of any type will slide around in the most amazing ways.
- Do you care if you mar the plastic tailgate cap?
- What type of tie-down straps you have available.
- You mention 8' boards, and what I'm writing applies to 8' lengths, or shorter only.
If I was loading on top of a closed tailgate, I'd ratchet strap across the tailgate end of the truck and call it a day. Between the wheel wells you should fit a 7 wide 4 high stack, with the remaining four boards centered on top. A good rope works for this method too, as long as you are capable of tying it off snug. The main goal is to stop the boards from sliding left to right in turns, or bouncing on bumps. Ratchet strap is better though. I do not recommend bungees. I know loads of guys who would load that many boards that way, and not even tie them down. And I've seen it done successfully more times than I can count. HOWEVER, I DO NOT recommend moving them that way. Tie them down please.
If I was loading with the tailgate down, in addition to the sliding left/right and bouncing up and down mentioned above, sliding out of the back of truck becomes something to take care of. I would make the same stack as above, but put two straps around the load itself, one in front, one in rear, binding it into a tight bundle. Then, strap the load down to the truck, including weaving the tie down straps to the binding straps in some way. Still don't go trying to light 'em up on take off, or put her on two wheels in a turn. I think ratchet straps are the best choice for this method.
For the 32 - 2x6x8' boards you described, I'd go on top of a closed tailgate if it was me.