OP, another option to consider, will the lender provide the same rate at 72 months? This would put your payment (roughly) at $440. However, make the payments as if it was a 60 (or less) month loan but you have some protection if your budget is running tight in a given month.
I understand and agree with Team Plutonium's point of going with a higher payment and direct more of the money towards the principle. However I don't know your budget nor is it any of my business (to know). Assuming that you plan on keeping the M3, at the current payoff level, knowing that after 5-years, there's still a $10k balance.
Have you applied at PenFed? Great rates. Per your statement on lack of installment loan history, I don't know how much this would impact PenFed (or another credit union) from approving of a refinance.