I am not going to claim to know with any degree of accuracy what weight Marciano could have bulked up/sweated down to. Frankly I have better things to do with my time than try to figure it out. However it seems inescapable, that what has worked for modern fighters, would have worked for him. I am certain that he could have come in significantly heavier, without sacrificing stamina or work rate, especially over the course of a twelve round fight.
It seems far more likely that Marciano would fight at light heavyweight than try to bulk up to heavyweight. Peds won't make him taller or make his reach greater
Why would you associate modern fighters Rocky's size with bulking up? The percentage of 5'10 guys who "bulk up" will represent a fraction so small of that size as to be basically zero.
He might be a good candidate to end up in that tiny fraction at some stage of his career, but that is not the case that I am making. The question being asked seems to be whether he could bulk up, and the answer to that is almost certainly yes.
So you have a limited 1 dimensional 5'10 fighter with the reach of a welterweight who was rarely over 190 even at his heaviest. And the usual suspects are suggesting he gain an additional 20-30+ lbs, do strength training, and eat lots of more food to compete with behemoths who have half a foot in height, a whole foot in reach, and would STILL on average end up being 20-30 lbs heavier? Cruiserweight be damned? Instead of simply losing 10 lbs and competing with light heavies where he can make the most of his best qualities: toughness, stamina, and a high work rate and where most opponents will not be dwarfing him or overpowering him in the ring? I am so glad none of you are managing fighters..
I suggest Marciano would have to adapt to the environment and I point to Orlin Norris as an example of what was possible within a Marciano type frame. if that means first establishing a foothold in a lower weight class first, so be it. Heavyweight might not be his lone weight class in more modern eras. The examples are there.
Could you provide three more? I'm most interested in fighters who are 5'10.5 or less with a generally short reach, like Marciano who did not turn pro at HW but who "bulked up" to get there.
My feelings on size and 'if they'd come along at the same time'. A big later boxer would, not necessarily but probably, have weighed less than what we actually saw of him, and might also be to various degrees shorter if he were a post'60's fellow transposed into a 1920's or earlier life. Vice-versa for the earlier boxers. But though I think a big latter boxer, his life transposed into an earlier era, would probably (himself) be not so heavy (and perhaps a bit shorter), I think the effect would be moderate enough that he would be more a 'big' fellow, relative to those earlier fighters, than he was 'big' in his own day. Now, if the scenario is, take a developed fighter from their own day, time machine, and they have to train by the resources and to the situation of another time, I think there would be some weight change, usually, but not so much as in a 'born into anther day' scenario. Well, that's how I fancy it ..... very speculative.
Orlin Norris Bert Cooper Dwight Muhammad Qawi James Toney. Darren Corbett Jean Marc Mormeck. Constantin Bejenaru There is also David Tua who on his debut was just 201lb which might have been 190lb or so in an earlier era given the weight weakening trends of the day. Eddie Mustafa Muhammad fought as high as 205 to take on Renaldo Snipes...he started out at middleweight. He was barely 5’11” George Scrap Iron Johnson was 5’9” and started out at 192 went as high as 224. He fought Bugner and Foreman around 217lb. These weights would put Johnson in two different weight classes in later eras. justin Fortune went as high as 248 and was just 5’9”. They have been around.
To be honest, Marciano was always an abnormality. There might have been plenty of 5' 10'' ish 185-190 ish heavyweights in his era, but you would have struggled to find other examples with a 67" reach. Also swarmers who are successful at the very highest level, are rare at heavyweight.