So, you're willing to go Sativa. Now, ordinarily I'd say go with a selection of Haze and/or C99, but in this case I'll have to throw up one of my own recommendations.

Malawi Gold is an African landrace Sativa, a pure one at that, and it's said to have a really strong high, very long lasting. The high is literally said to be equal to most Hazes. Coincidentally, the flowering period on it is like a minimum of 90 days. Should be worth it, though.