Are we talking about a hypothetical prime for prime match on the date mentioned or is it what we think could have happened if Frazier had made a comeback on that night? Joe was wanting to fight again in that year but a bout of Hepatitis put paid to it.
I think Leon would be too much for Frazier at this point. He'd still be riding high off of his thrashing of Ali, hopefully he would stay focused and train properly and I think he would have made a damn good account of himself against a shop won Frazier. Imagine how we would look at him today, regardless of if he still lost his title right back to Ali, going 2-0 back to back against Frazier and Ali
So the way I usually interpret these, when there's a date given and no mention of "prime for prime", is for each fighter to be "as-is", give or take some common sense modifications. (ie, in this scenario, Frazier remaining active after the Foreman loss and having at least a bout or two in 1977 rather than coming in here directly off two losses followed by twenty months collecting rust).