Frame geometry

I have a frame that I bought as a Colnago but is not. It is still a very nice frame from Tange tubing and a nice lug set and I was actually wondering what bike to build from it. Measuring the size of it, I was a bit surprised to find that the size is 58 (c-t) but the top tube is only 54 (c-c). I have seen this relative short toptube before on Japanese bikes (Nishiki).
I normally like a toptube of around 58, so I am a bit doubtfull if I should proceed. Was it originally maybe a touring frame? It is probably from the first half of the 80's.
What should I do? O, by the way, I was actually planning on building a tourer with a Nitto moustache bar. As these mostly require a shorter stem, maybe a shorter frame will work as well!

I hope you didn't pay Colnago money for the frame :shock:

Sell it on.
Sounds far too small.
I have got a nice 58cm square steel road frame for sale....

That could certainly be an option. I didn't pay very little, but it included a near mint 600EX set and 2 wheel sets, one with mavic rims on old style Dura-ace hubs and one with Campagnolo rioms on dura-ace rims. So I was pretty satisfied anyway.
As I have the frame without headset and BB at the moment, I thought it was a good moment to weigh it, maybe to determine if it is made from Tange #1 or Tange #2. The frame only weighs 1875 grams!! I have never seen or even heard from such a low weight for a steel frame.
I can't sell it on, before I know what it is.