A '58 Veloce will definitely need DC03 carbs..... big $$$ on their own unfortunately..
Veloce blocks are difficult to find, they do come up from time to time, if you do perhaps find 1315*30800, it's the original block for my '58 SV & I'll swap for the '58 SV block I do have.
On another note, I commissioned a run of the 750 Veloce tapered tube water rail in stainless steel, powder coated in satin black - better than new !
Ciao
Greig