I happen to like the 79-80 cb750F front fender also as it hugs the front wheel nice and close. I had one I bolted to a SOHC fender mount. The Fender is plastic so it will need some kind of brace underneath. Currently it is in my attic and I didn't have much road time on it before I decided to ditch it all together. I never tried to bolt the fender to the stock forks, but from what I can see the front forks on my brother's 79 DOHC F and my 78 DOHC F are pretty similar.
Stay away from the later 81-82 front fenders, that little spoiler on the front fender looks gay on these older bikes.