After modification of the gun to accept an extended magazine, you can easily return it to the standard 4 shot magazine.
All you need to do is buy an old style magazine spring retainer, and use an O-ring or a special washer from Brownell's to prevent the magazine cap from unscrewing.
Install the original 4 shot length spring and use the retainer to hold it in place. Put the O-ring or washer over the magazine tube after you install the barrel and screw the original cap on.
The barrel's mounting ring will cover the holes in the magazine tube where you drilled out the tabs.
The old style magazine retainer is a spring steel split "spring cup". It goes in the end of the magazine cupped side up.
It's part number 38 in this Brownell's schematic:
I think this is the washer, or you can buy a large O-ring at an auto supply store:
When removing the tabs in the Express magazine tube, make sure to polish the inside of the area very carefully. Even the slightest burr can cause shells to stick.