If Galento had replaced Ali in the 1960s, could he have reached a title shot against Liston?

No.

Tony Galento was an average puncher compared to Tua. Galento had 111 pro fights and only scored knockouts in 56 of them (basically half). And two of those KOs came in fixed Boxer-Wrestler fights at the end of his career. (So, less than half.)

Tua knocked out four heavyweight champions (Moorer, Ruiz, Maskaev, Rahman) ... stopping Moorer and Ruiz inside 30 seconds. Galento never did anything like that.

Tua was an Olympic Bronze Medalist and a dominant heavyweight for years, only losing three times in his first 57 pro fights.

Again, Galento was never that. Ever.

I know some "hipsters" like to point out Galento was briefly ranked #1 by Ring, but that was comical. Louis had already torn through most of the top fighters on his way up and in his early reign, and it only shows how awful the remaining options were.

Galento was a member of the "Bum of the Month" club because, even back then, he wasn't considered a threat.

Lennox Lewis may have defended against a couple questionable opponents, but David Tua certainly wasn't one of them. And Tua was never considered a "Bum" at any point in his career. Even at the end.

 

Galento beat some of the best of his era. It could be the case that Galento fought better fighters than Tua did.