So I have this new production 590SP, that I want to put the old style wood furniture on.

As I understand it, all 500/590 series buttstocks are pretty much interchangeable.

What I'm worried about is the fore-end. Like I said, mine is a 2014 production, so I need the double-bar type pump?

The 500 and 590 fore-ends interchange too, don't they? Provided it's the proper length, which mine is the longer 7.75" one.

Yes, with the exception of .410 buttstocks and the new Flex series buttstocks.  20 gauge buttstocks will fit a 12 gauge receiver, but the receiver will be slightly proud of the buttstock (hardly noticeable).

Yes, you have two action bars, so you will need a fore-end made to accommodate two action bars.  Only the early 500s had a single action bar.

500 and 590 fore-ends interchange within the same action tube length (there is also a 6-1/2" length in common use) and gauge (there is no interchangeability between gauges).  You'll need a 12 gauge fore-end made for the 7-5/8" action tube.  Due to all the variations in wood fore-ends made over the 500's 50+ year production, you may need to rely upon the seller's description.

If you haven't found it already, go to the "Modern Shotgun Stocks" section:  http://www.havlinsales.com/