Leon Spinks got tremendous mileage out of his amateur credentials. As a professional though, his only major world class win prior to Ali was the close decision over Alfio Righetti, which led to that title shot upset. Evangelista and Mercado were easily his best heavyweight post-title wins. The Mercado stoppage proved that he was a very legitimate heavyweight. Yes, in 1911 Willard reportedly had at least 20 total bouts, including exhibitions (out of which boxwreck currently records eight official outings), 13 in 1912 (of which boxrec records seven), and 13 more in 1913 (out of which boxwreck records 11). Aside from Righetti, LeDoux was the only substantial heavyweight contender Leon faced prior to Ali. Willard had the names of Pelkey, Morris, Gunboat and McCarty on his dossier. Soldier Kearns, Dan Daily, George Rodel and George Rodel were all opponents with winning records who Jess had posted stoppage victories over by the time he challenged Johnson. Post-title, Willard's heroic comback TKO of Floyd Johnson was a huge upset. Fans in the crowd of nearly 63,000 at Yankee Stadium laughed when Willard's weight was announced as 248. 11 rounds later, the Auburn Bulldog's title shot at Dempsey was out the window.
Who are these solid contenders? I'm guessing you'll have to come up with very obscure names as "solid contenders"; his most notable opponents pre-title were Smith and McCarthy, and he beat neither of them. He also lost to journeyman lightheavyweight (remember that Willard was 6'6 235lbs) right before his title shot. Pure white hype. As for his title winning effort, he got thoroughly schooled through the first 15-20 rounds, and merely won because he was younger, focused and in good shape, not of which the disinterested, from USA banned Johnson was at that time. Under normal gloved boxing rules, he would've lost a shutout decision to that same old man, who had been in long and frequent battles since his teen years. Certainly a top contender for worst record before winning the title.