The main concern when adding a magazine extension is the thread engagement.
The 90 degree flat threads on the end of the magazine tube (connected to the receiver) need to be utilized along their entire exposed length.
Remington factory, Vang, and Wilson/Scattergun (and some others) have the provision for sling attachment farther past the junction (forward).
Positively avoid any sling provision that acts like a "washer" between magtube and the extension.
There are too few threads remaining to secure the extension.
The threaded portion was initially designed to merely hold a cap, and not an extension. There are enough threads to do the job, but just enough. You will also note that the factory extension has a clamp assembly to further reinforce the connection.
You do not have to run a clamp assembly with Vang/Wilson/Scattergun if it is on nice and tight.
One more thing- On a Remington factory extended magazine shotgun, a captive detent is installed on the magazine ring installed on the barrel. The 870 Express etc. has the hole for the detent, it is just not installed. You can purchase the detent and spring from Brownells and install it yourself. It really will make a difference.
Remington used that plastic ratchet spring endcap to prevent the metal domed endcap from rotating loose, but once you forego that assembly, you really should install the detent.