Short Reach handlebars or Heli-bars might be your answer, but before heading off down that track, try making a conscious effort to RELAX your grip on the bars, drop your elbows down and your shoulders back, and instead of actively 'steering' your Spyder in response to all the little movements/minor deviations that you might feel as a result of having such a tight grip feeding on so much more tire contact patch, especially up front, than you probably never had on a 2 wheeled bike, simply try to look further ahead down the road as far as you can see, choose your path, and then with a loose grip on the handlebars, just gently guide it down said path without reacting to those little movements.
Many, if not actually
most 'new Spyder ryders', get this 'pain between the shoulder blades', at least initially, and
especially if they've come from years of 2 wheeler experience, they tend to go thru this 'hang on too tight and try to force the Spyder to respond like it's on rails' or like their former 2 wheeled rides did, but Spyders don't work that way! Most have found that by relaxing their grip on the bars, dropping their elbows down, and their shoulders back, then looking waaay out in front and gently guiding their Spyder basically down their chosen path but without responding to all the little deviations caused by the extra contact patch and the lack of the need to counter-steer, the looser grip etc ends up relieving that shoulder/back pain.
Sure, it doesn't work for everyone, and short reach bars or Heli-bars might still be needed, but if you look at the number of older threads here (going back to 2008!) that are talking about this shoulder/back pain issue and the way relaxing their grip has relieved so many over the years, usually
without resorting to changing the handlebars, you juuust might be surprised, and then maybe even more pleasantly surprised if it works for you too!
Give it a go, who knows, it might save you some time and a bunch of $$, as well as a heap of back pain!