Well, I'll do my best to tackle this one...
Dropout spacing is the same between Campy & Shimano; I believe its 135mm - That part is standardized.
Also, Yes - Shimano MTB cassettes can be installed onto Shimano (or clone) road hubs.
In the past, I have used an Ultegra (8 speed) cassette on a White Ti hub, as well as an XTR (12-28 I believe) cassette on a road wheel.
I will tell you the shifting was not quite perfect in either case - Just annoying enough to be distracting. Although, there may have been other factors such as less-than-new chains, and less-than-new cassettes themselves. I suspect that a brand new cassette, in conjunction with a brand new chain should shift & work quite fine together.
So, you could, conceivably, install a Shimano road wheel, with an MTB cassette to get your desired gears.
There are, however a few significant problems with this...
First, and most significant is the shifter/cogset spacing & compatibility- In other words: How far do the Campy shifters move the derailleur vs the exact distance between the rear cogs.
I do not know the figures off hand, but you could research that part.
*If* the spacing the same or very, very close, it *should* work, but they simply may not work together when assembled & actually ridden in the real world.
You might then need to add Shimano shifters to get it to work - Pretty much a mutt-bike at that point!
I think you realize that there's just no putting a Shimano cassette on a Campy hub - So don't even think it! 😉
Another option:
I believe your San Remo has Campy Mirage or Veloce components right?
The simplest fix would be install a triple crankset.
No problem, Campy makes several triple cranks in either 30-40-50 or -30-42-52 chainrings, but there's a hitch...
Most likely the O.E. derailleurs are of short cage/twin crank specs, so you would also need to replace both derailleurs in addition to the new crankset.
You *may* also need a longer BB to work with the triple cranks - But not always - Check with Campy or a good knowledgeable Campy source to confirm.
By the way: No, You cannot use Campy BB's with Shimano cranks or vice-versa.
The length and angle of the flats is different. I believe Campy uses a 1-degree bevel with long flats, while Shimano uses shorter flats & a 2-degree bevel.
Completely incompatible at any rate.
So, a couple compromises & choices.
I've mixed & matched many variations of cassettes, hubs, derailleurs & shifters, but never cross-bred between the two species.
I just don't think you'd have much success getting a Shimano cassette to work acceptably well with the Campy shifters.
My suggestion would be to install Campy's largest cassette (13-28 I believe), and see if that does the trick - Quick, simple & cheap.
If you still need to go lower, I think you are looking at a Campy triple crank & replacing both derailleurs (& possibly the bottom bracket).
*Unless your O.E. derailleurs have the capacity for triple cranks - Check you owner's manual, or research on Campy's website.
No easy cure - Best of luck!
