A 26" tall tire is about equivalent to a 295/50-15. So that means yesterday. Perhaps the email address was put in wrong? I ran 3.91's with 295's and it was liveable at 60-65mph, and fun "light-to-light". 3.73's might be nice too.
As for convertor, there are too many factors at play for that recommendation, and a good convertor manufacturer is gonna ask a lot more questions than what you've disclosed.