I’d favor Frazier. Shavers got sparked the same calendar by quarry as Frazier did to foreman. Earnie didn’t have the type of two fisted attack or size advantage over Joe.
Joe's fundamentals were fairly good enough to not get caught flush. So he would have outworked Earnie and probably TKO him rd. 3-4. I think 80% Frazier / 20% punchers chance. In fact i will favour him over any big puncher in the 70s (Lyle, Cooney, Weaver...) except Foreman.
According to Eddie Futch, The Sunshine Showdown was Frazier's career worst performance. In THE EDDIE FUTCH INTERVIEW he describes Joe's training camp as "one great big party." Sure, he did train as was expected of him, but he was mainly going through the motions. Ken Norton who was part of his camp at the time outright stated that Joe seemed to have lost his drive. That being said, I would still pick Frazier to win. In my opinion, Earnie Shavers was never anything close to George Foreman in any shape or form when it comes to style. The one thing they had in common was power and while Frazier's style made him somewhat vulnerable to big punchers, this weakness was nowhere near as pronounced as people make it out to be (Futch developed Joe into a bob and weave fighter after the first Bonavena fight specifically. The one that is often used as a proof of Joe being weak against punchers.) 1971 Joe would probably laugh at most punchers' attempt at setting up the big shots. Check The Ring statistics for % of punches landed on Frazier in his prime years. Sure, each fight has different set of circumstances, but it makes you think. What really hurt Joe against Foreman, bar being ill-prepared, was: a) Foreman's battering ram of a jab, or a sledgehammer according to Quarry. It was hellishly strong, it was deceptively accurate. There was a reason it was his most trusted weapon during his second career. Watch George's 1968 Olympics fight against Jonas Čepulis. He massacred him with the jab alone and he was just an 18-years old boy with a year or so of real boxing experience, going against 29-years old experienced boxer who won Lithuanian titles (1963–1964, 1966–1967) by the time he came to fight in Mexico City. Foreman's fight against George Chuvalo is another example of what his jab could do to the other fighter, shorter one in particular. b) Foreman's control game. It was superb. During Foreman's fight with Dino Denis (October 15, 1976) Jerry Quarry calls George the most awesome physical specimen in the heavyweight division, but more importantly during another fight with Scott Ledoux (August 14, 1976) he describes his power as his defense. George used a large variety of old school techniques like leverage guard/stops (stopping an incoming punch actively or preemptively by putting the glove on the biceps, elbow joint or shoulder) or muffling (stopping a punch before it is even thrown by putting your glove in close proximity to the other fighter's fists; literally a catch but done preemptively.) On top of that he was trained in what was referred by Howard Cosell as pushing tactics, in reality a rough but surprisingly elaborate form of control game: Foreman would push, put a glove on the shoulder of a ducking fighter that tries to come low, or outright move his opponent to the side, confusing him and forcing him to reorganize his offense. He often roughed opponents up during clinches as well. Pretty much what you would expect from a man trained by Dick Sadler and Sandy Saddler. c) Foreman's variety of punches. I feel like that's another thing that is often overlooked when talking about George (bar his impressive defense/reflexes.) When his opponent was disarmed and/or cornered, he would launch an offense of hooks, uppercuts, and shovel hooks. Earnie for all his power was mainly dangerous because of his right hand. Bottom line, Earnie Shavers had the power to hurt and potentially put down poorly prepared Frazier, but I doubt he is nearly well equipped and prepared to set Joe up and land flush like Foreman did.
This video is the perfect explanation on why Foreman was able to dominate Frazier, and many heavyweights in his prime and why the old saying "Styles make Fights" is so important in boxing. Foreman was taught to take advantage of his greatest attribute. great physical strength, possibly the strongest man to ever box at a world class level. and he used it to it's maximum effectiveness. The pushing, and pulling he used to set up his power punches is something uniquely his own. Though their were bigger men prior to Foreman, and many after him none used their physicality the way Foreman did. Many posters on this site think any large fighter would have the same success against Frazier. They don't take into account the key to Foreman's victory , "The style". for instance to think Lennox Lewis would do to Frazier what Foreman did is a very flawed point of view, he simply didn't use his physicality the way Foreman did. Not saying Lewis couldn't or wouldn't beat Frazier, just saying if he did, it probably wouldn't be as dominate as Foreman, and it would be in a more conventional way. Shavers would get stopped by Frazier between 5th or 9th rd. If prime Frazier could make Ali miss the vast majority of his punches. I see no reason he wouldn't make the much slower hands of Shavers miss, and make him pay for it. Personally I don't believe the fight would be as close as some seem to think.
An excellent points on Sadler and Saddler., if their was a fighter/trainer in history that could teach Foreman to use all of his physical ability to it's maximum effectiveness ,it's none other than Sandy Saddler. Possibly the most physical fighter in history below welterweight.
Dick Sadler aside: Foreman was shaped by Sonny Liston, Sandy Saddler, and Archie Moore. That's helluva training regiment and it is always a great joy to see their craft live through George at different stages of his career.