JasonS, get some poly straps. The Kammok Python straps are great & the ENO Atlas straps look good too.
If you can't take the Slap Straps back (REI's great for this!), then you can 'pre-stretch' the straps. The other thing is, the more strap length in use, the more stretch. For instance, if you only have 1' coming off each end, because the trees are so close, then you won't have much strap to stretch. If you have 8' on each end, then hang the hammock up so that it's straight across the spread, then hang across the hammock for a couple of minutes. The straps will stretch. Re-set the loops up higher, then hang out for a while longer, again re-setting up higher. You can take out a couple of feet of stretch this way. Just remember that the line will shrink back later.
I made the mistake of putting two slap straps together one time at the loops. The stretch really synched down on the loops. When I took the hammock down, they contracted. It took me over an hour to work the loops free again.