I’m 6’ (and change) tall and ride a 56 cm TT on most of my bikes and a 78 cm BB center to saddle top. But the geometry and “size” of my bikes varies a great deal between them. As an example, I have a 48 cm (CT) gravel bike with a long seat post and a Cannondale R900 60 cm “size” which is, for me, a size too large. However, they both have 56 cm TT sizing and I run a 100 mm stem on the gravel bike and yet a 120 mm stem on the Cannondale. So fit wise, the Cannondale would require me to go to a 130 or possibly 140 mm stem if I sized down to a 58 (which I’ve been trying to do). The gravel bike could easily be 50 or even 52 cm (CT) all else staying the same essentially reducing the slope of the TT and I’d just run 2-4 cm less seat post extension.
All this to say that judging by the latest photos, yes, a 54 Cannondale of that vintage is likely too “short” for you at 6’. Particularly with that stem/bar combo. You should measure the distance from the center of the ST to the center of the HT along the TT. This will give you the bikes “length” in terms of fit. By the length of the stem (70-90 mm?) in your side shots, I’d say you’re probably 40-60 mm too short on the stem. Maybe more. While bodies are different at the same height, for reference I run a 58 cm length saddle nose to center of bar (center of stem at bar mount point). This is true on all my bikes +/- 1-2 cm depending on the reach and drop of the handlebar. Some bikes I run a 100 mm stem (gravel), some a 130 mm stem (road aggressive). But another thing to keep in mind is also your saddle to bar drop. In your photos your stem is extended “out” of the headset quite considerably and yet your seatpost extension is not substantial. So the frame I think can be made “to fit” in as much as many 6’ tall pro riders ride 54 cm frames.
So given your seatpost extension, I’d definitely say the bike fits but your bar reach is likely way too short. It’s also rather high making it feel shorter yet and causing you to arch your back too much to “fit into it”. I’d venture your saddle to bar distance is about 52-54 cm vs my 58 cm but it’s just an optical guess. So you’d need to go longer. How much? That’s rather individual but you’d likely be somewhere in the 55-60 cm range length depending on a number of factors (body, age, flexibility, experience, arm length, etc.)
Take a tape and measure it up if you have no other “comfortable bike” for reference and then compare those numbers to the ones I provided. I strongly doubt you’d be comfortable and efficient on anything less than 55 cm length but that can be adjusted with seatpost setback (though I prefer a forward position with zero setback on most road geometries with ~73 degree seat tube angle), saddle rail position, stem length and stack as well as bar reach and drop.
Those vintage “Cinelli style” bars tend to have deep reach and drop which is why you likely have your stem so high. Going to a longer stem with a lower stack and a slight upward angle would put you in a more agressive stretched out position. This along with a more modern bar geometry will feel “awkward” at first if you’re not used to road bikes but eventually will feel more comfortable and optimal for handling a bike like this.
We should probably both be on 56-58 cm frames from this era. I’m fine on my 60 cm frame given the 120 slammed stem I run but I would prefer more seatpost extension for additional compliance there. I’d also run a carbon seatpost to absorb more road rash in these otherwise super stiff frames. In short, my optimal sizing would likely be a 58 frame with a 130-140 stem and a cm or two of stem spacers to increase stack. Or maybe I would still enjoy it slammed as well.
Let us know what numbers you come up with and we can probably give you some further tips to improve the fit.
Of course, you could also get a professional bike fit and have them assist you to see if you can get this frame to work for you.
P.S. One thing I did on my R900 was to switch to a threadless carbon fork. This gives you many more stem/bar combo options while also providing a more comfortable ride up front. The aluminum sub one forks of this era were very stiff and not very kind to the body. Eventually, Cannondale did the same and ran stock carbon forks on later 1990’s vintage bikes.
Likewise on the handlebar. I’m a heavier guy at over 200 lbs so I liked the stiff bar but prefer running a carbon bar now with the switch to the fork as well. Lastly, just as an FYI, the retrofit R900 went from being a 9.5 kg stock bike to a 7.5 kg ride. That, with an electronic rim brake groupset (heavier derailleurs but lighter cassette, crank & BB) and 50 mm deep carbon wheels with 28-30 mm tires. Yes, I can fit some 30 mm tires on these wheels with the new fork. The old one only passed 25 mm measured tires on the stock old Mavic Open-4 rims with 15 mm internal. The most critical area for tire clearance is the junction of the chainstays to the BB. No problem passing 32’s at the frame brake bridge or at the new fork crown. I did have to use a longer reach brake caliper up front with the carbon fork. Also, the fork geometry is rather critical as you don’t want to alter the overall geometry too much and you have to be mindful of toe overlap and wheel/tire clearance at the downtube. Basically, if you keep the rake and axle to crown metrics the same, you’re fine fit wise with any fork you like.