If the trees are too close there's not much you can do about it, it's one of the only downsides to using whoopies IMHO
Why not get some Dutch Buckles on your webbing so you can at least attach your whoopies right up next to the bark if you need to.
Get some longer huggers if the trees are likely to be chunky, I have to use 12' straps in some parts of Scotland. Get some poly webbing and make a few sets up, sewing one loop by hand for a Dutch Clip and then slide your buckles on.
Buckles will help you cinch your straps right down, you'll only be restricted by the double layer of the sewn loop at the end, by then the trees are probably not safe anyway.
A marlin spike hitch is the other option if you don't want the Buckles.
I sound like an advert for Dutch but the Clips and Buckles have been some of my best buys of late, as well as his Flyz and Hooks