M2 Field question 80087 LE +2 Extension

[metroplex]

I have a M2 Field and wanted to know if anyone has retrofitted the 80087 LE +2 mag extension. 

do I reuse the stock mag spring or do I need to trim it?

does that swivel freely swing around possibly hitting the barrel?

are there any instructions posted? I saw photos of the packaging online and didn’t see any indication a sheet was included.

ive been using a Nordic +5 tube with a Nordic clamp for 12 years. But I haven’t been a fan of having to disassemble the barrel clamp and removing the long tube and the long spring. the factory +2 seems more convenient.