In this video, I show the process I went through to figure out the correct 'solution', using parts included with the Shindaiwa "Speed-Feed Universal 400 Trimmer Head" in order to fit it to my Stihl model FS90R string trimmer.

I show this because the FS90R is NOT one of the great many trimmer models listed by Shindaiwa as being compatible with their 400 trimmer head, and so there was a bit of messing around and experimenting to find the correct combination of fittings to make the match work out.