...it appears you now have a direction.
Some observations on Evapo-Rust and on OA baths. Both are dependent on warmer temperatures to work more quickly, and certainly OA is not something you want to be using indoors. So since we're getting on into fall and winter now, you might want to time this so you don't end up doing it at the wrong time of year.
As @
juvela has already said, you need to appreciate how much rust is probably accumulated on the inside surfaces of the frame, if there's that much corrosion on the outside. I think what happens is that these olde bikes get stuck in an unheated garage or shed, and as they heat and cool, the moisture in the air condenses on the surfaces (interior and exterior). And depending on the relative humidity and temps in your location, over the years everything gets pretty crusty. So either complete submersion in OA solution (OA is cheap), or some people have had success using a large, heavy duty plastic bag, inserting the frame and fork, and then using bricks and other stuff to arrange the bag so the Evapo-Rust (not cheap) goes inside the frame tubes and stays.
I've done both, and can't really say I prefer one over the other. The OA is more obnoxious to use, and wear gloves because it gets absorbed through the skin. OA crystalizes in the kidneys with bad results. Also, youi need to neutralize it with baking soda or another base before you dispose of the liquid when you're done. It's a better operation if you're doing more than one frame at a time. But you are not.
I certainly understand the preservationist aesthetic in bike restoration. I have a friend here, Jack, who is unsatisfied if his bikes don't have the original tyres on them when he gets them.
I can only thank Jack for eventually selling some of them to me (we ride the same size). I suspect I put a lot more miles on them than did he. But it's a big ol'world, and everyone has his own vision of what makes him happy in this hobby.
It occurs to me that with a Frejus that old, you might want to check your frame tubing diameters. It's remotely possible that those newer Campy parts you were thinking of using have clamps that won't fit your frame tubing, but it's a quick and easy job to measure. And if you're willing to clear coat and treat the frame interior after you get the rust under control, you ought to be good to go for quite a while. It helps if you treat a project like this one as a long term project, with no clear ideas on turnaround. I think it took me well over a year to get that Carlton Flyer sorted out, and it had no rust issues to speak of.