- Leaving it on the pallet is fine. I've done it with 2 compressors with no movement or issues. I trimmed my latest pallet down with a sawzall so it wasn't so large.
- Extend the drain like Markcuda shows with extension/valve/turndown elbow. I've done the same thing. Make sure the pipe you use is rated enough for the full system pressure, but that shouldn't be hard to find as it should be under 200psi.
- IF you bolt it to the floor, you should not rigidly mount it. The tank will expand and contract as you add pressure or drain it down, and if the feet are too rigidly attached, you can stress the tank where the feet mount. Rubber pads or mounts are a good idea for both expansion allowance and vibration isolation. Manufacturer may require it in the install instructions. You can also do this by screwing one down tight (over rubber pad) and leaving the other two slightly loose with nylock nuts, for example.
- Always use a short section of flex hose from the compressor to hard piping. Makes install/maintenance a billion times easier and isolates vibration from your hard piping.
- Run horiz piping on a slight down slope with a drain leg/valve at the far end to trap moisture. Where you want to tap into the line for air hoses, tap into the top of the pipe and point them UP initially so that condensed moisture does not run down into your hoses and tools.
- Soldered copper ($$) or iron pipe work fine. PVC is a major safety hazard when used for compressed air no matter what anyone tries to tell you.
Those are the key things that are top of mind.