It's been a few years, but I've run the numbers before to help out a retiring friend. I won't go through the entire background, but I'm not a fan of SBP due to the many sunk costs with minimal gains. If you're retiring in your 40's, the following plan works pretty well. I recommend taking the same dollars that would go to that 6.5% and putting part of them into a good 20 year term life policy. Take the remainder and invest it according to your preferred strategy.
If you die in the next 20, the SPB income your spouse would have received is replaced by the insurance, with possibly more. If you die after the term expires, you have specific investments to support her living in a similar way. By that time, with conservative figures, there should be enough invested to exceed SBP benefits if paid out through an annuity.
When I ran the numbers, it came out to 20-40% above SBP benefits in almost all cases where you die before her. Obviously if she dies before you, it's worth 100% more since you would otherwise get nothing. Caveats: there's more risk in my plan, and I personally have not retired yet.
Edit: Subject-verb agreement.