I saw the stuff you found, (#18 yellow right?) Looking for some myself. Put small lightweight straps around the tree, Mini S-Biner, then hook the mason's line to the S-Biner. Done.
Or... You have 400 feet, use it up... LOL
I found mine at Lowes. I want to say it was 500' for about $6.
With the braided masons line, if you have lots of time and patience, you can actually make miniature whoopie slings. Its harder than amsteel because its so small!
Maybe I'm not working the Amsteel right, but burying the Mason line in itself seemed pretty easy. You just have to "fluff" it before trying to put the needle in it. However, I found that to keep an adjustable loop that would hold decent, I had to bury at least 12".
I used mason line when I first started hammocking. I was much happier when I found better alternatives like Zingit, Speer No Tangle, Kelty Triptease, etc. All less tangle prone and more abrasion resistant. Also the colored mason line leaches dye bad too. Try throwing a spool of it in a bucket of water and watch it turn to koolaid colored water. It will stain your tarp. My MacCat still has pink stains on it.
Maybe there's different types of Mason line?? The stuff I found at Lowes is yellow and looks like it might be braided nylon. I haven't seen any color bleeding and the 10' I keep bunched up in each of the corner pockets of my tarp just seems to fall into order when I pull it out. No tangles so far. maybe it gets tangly after a long period of use. I've only had it on my tarp for a little more than a month. But at the price, I have no qualms replacing it as needed.