I often use Mutants and Death Ray Guns for 'straight' sci-fi. I do have Flying Lead, but haven't played it for years. It worked pretty well when we did play it, but MDRG is a bit simpler and 'cleaner', as I recall.
It's easy to remove the post-apocalyptic elements from MDRG by just disregarding the random generation of characters and the weapon-jamming (and armour jamming). There's a great warband generator on the Ganesha site, so you can just use the points system and stat up whatever you want.
The Simian Ultra X supplement adds the reaction trait from ASOBH. Of course, you can just add it without the supplement, but it's well worth getting for the new weapons, species and equipment. Most of those are 'generic' sci-fi rather than specifically post-apocalypse.