You have a point, here... However, Monzon would still be at a considerable size and power deficit, and would at the least not enjoy the dimensional advantages he usually did. Monzon did possess incredible durability, physical strength and endurance- but would his strength, which was such an asset against men his own size and smaller, hold up against a big light heavyweight like Foster? The same goes for his chin- it was unbelieveable at middleweight, but against potentially the biggest puncher of all time at light heavyweight, would it still prove so undentable? If Monzon can effectively withstand Foster's punch and is capable of physically holding his own with Foster when they get into the rough stuff, then I think he has a good chance at winning on points, particularly if he can wear Bob down late. But the thing is, I doubt those two premises would stand up- Foster would probably be able to fairly readily hurt Monzon and physically bully him, which would likely lead to Monzon being broken down into a stoppage.
Monzon did have a great chin but Foster's power at 175 was frightening. Foster was taller, stronger, harder hitter than Monzon and almost as good a boxer.
I agree with what many has said here. Foster was a great LHW while Monzon was a great MW. A good big guy always beats a good little guy, right? Which brings us to another question. Monzon never aspired for anything in the higher weight class. So, why on earth would he want to fight Foster at LHW? Now, if Bob came down to MW (heh, heh), that would be interesting.
So let them meet at a catch weight of 166. Then it's closer. Although I don't know if Foster could fight at (i.e., make) such a weight. I'm sure Monzon could. There was no super middleweight division back then to allow for gradually moving up from middleweight. Middleweights tended to stay there because most couldn't compete with the light heavyweights. Although some like Tiger moved up. Also, all fighters back then were actually about one weight division lighter on fight night than their counterparts of today. Monzon fought at a fight time weight that welterweights fight at today because of the early weigh ins today. Pavlik looks bigger than Hagler and Monzon and that's part of the reason. What's his fight night weight usually?