Long definitely seems to be the weakest link in attack. With the likes of Bale and Mané out wide, Tadić could be a seamless fit as a
False 9 (though you could argue that he doesn't qualify because he started flourishing in that capacity at Ajax under Ten Hag). Structurally, there's not much between what Southampton could hypothetically field and the template Klopp employs at Liverpool — Mané, Oxlade Chamberlain and Lallana have all at least done well at various points under him, Bale is in his natural habitat as a right sided forward, and Tadić would emulate Firmino...
As regards the primary “could Southampton have won the league” discussion, I'm tempted to say no, they would probably peak at Tottenham's level in terms of being good but not quite good enough when all is said and done. Qualitatively, the forward line and central defensive pairing is fabulous — but the midfield doesn't seem to be strong enough to come up trumps in a rigorous title challenge, and the fullback spots leave a bit to be desired too. Consistent Top 3-4 under an appropriately competent manager, like Pochettino — if he is also retained, would be within the realms of possibility, though — with maybe a
perfect storm year of them launching a proper, but ultimately unsustainable, title bid.