It is 5/32 hose, and is not carried by most auto parts stores. I did not know that trucks used it....that's good information.
On the old frame bikes, you did not have to run the tubing near any hot items because the compressor/solenoid assembly mounted underneath the battery box. On the newer frames, the assembly mounts between the frame underneath the bike, so when you route the lines, you must be careful not to get it next to the exhaust. Just for good measure, I'd suggest using some kind of heat protective sheath or tape. I ended up putting heat reflective tape on mine, then running it inside a piece of shrink tubing of appropriate size for the part that comes up the frame and then underneath the seat. Then it is protected against both heat and abrasion.
The HD air lines will work, but the tubing is not quite as durable, IMO. It's "softer".
When you plumb the system up, I would recommend leaving the connector behind the left saddlebag used to air up the old HD shocks "just in case"...you can then go by a gas station and at least air it up if the compressor were to go belly up. The little HD hand pump will not provide the volume needed to pump up the Legend's boots...at least not from my experience.
Keep a small roll of about 10' of tubing in one of the lower fairing glove boxes...it takes up no room. Another recommendation if you plumb the tubing up to the original fitting....make up a short 6" or so piece of tubing with a plug in the end (find a nail or other suitable sized item to use for a plug...it must be a really tight fit. Heat the end of the tubing if you need to). That way, IF the tubing bursts going to the compressor, or any other malfunctions occur, you can isolate the plumbing by disconnecting the line going down to the compressor and install the line with the plug in it, then use the original valve to air the shocks up. The weak link in the new system is routing the tubing well and protecting it from the exhaust or other hot things. The Legend Boots on the shocks are Kevlar reinforced, so they are unlikely to give any problems...if they do, it will be a very slow leak. It happens, but not often. Anything mechanical will wear over time though.
The customer service at Legend is excellent, from my experience. Rumor has it that they are designing a shock with a rebound damping adjustment, but I'm not sure how far along that is, or if it is ever going to come to fruition. The newer shocks have a larger boot than the older versions, providing more volume, thus more "cushion". Those with the older boots can be upgraded for a slight fee.