@Grizzly If you really want to do the tax stamp thing you are not going for an SBR you will be going for an AOW (Any Other Weapon) or and SBS (Short Barreled Shotgun). Your shotty as it stands is a LEGAL firearm as it does NOT have a stock, it has an arm brace. In this world, correct vernacular IS important. For you to pull your “Brace” off and replace it with a more substantial “stock” would be ILLEGAL. To pull your SB4 off and to replace it with say a KAK Shockwave 2.0 would be LEGAL as it is a “brace”. Only those items declared by the BATFE to be “pistol arm braces” can be used.
As to incidentally “shouldering” a pistol brace it is LEGAL as per the ATF letter dated MAR 21, 2017. HOWEVER the devil is in the details and you have to navigate your way around all the legaleeze. The take way is that by using a “brace” you cannot “modify” it or change it in any way other than as approved for sale by the ATF (that includes taking off the Velcro) as that would constitute a “redesign” which would be in violation of NFA. When surfing the web for NFA information it is IMPERATIVE that you pay attention to the dates of the letters and articles as things change over time.
Hope that helped.