Are the barrels interchangeable between the 2. I am looking to SBS the 590a1, and don't want to buy another barrel, due to being taxed 200$ for the form 1.

Mag tubes and hand guards are different lengths. You need to replace those also.

The barrels on my 14" 590A1 and my Shockwave interchange.

Are you thinking of putting the 14 in shockwave barrel on a non NFA 590? It won’t fit unless the 590 started out as an SBS.  A 590 14 in barrel will fit a shockwave. The mag tube on my 18.5 in 590a1 is about an inch to longer than the mag tube on a shockwave.  So you will need more than just a barrel to convert a 590 to an SBS. Obviously don’t make an SBS until you get your form one back.