Even for someone of your proportions, a 27" wide boat would be HUGE. The widest single sea kayaks are typically in the 24"-25" beam range and they will handle someone considerably larger than you. Either way, you're not going to be able to modify the Guillemot design to suit. You need to find another boat that's specifically designed with a wider beam. There are several out there, but they're not going to have the look of a Guillemot, nor will they have the same performance.