I have a miroku 3" (Charles Daly) that lost a barrel I will measure its monoblock and see how long it is.

As for question #2- Not all 3" guns have 3" monoblocks, some manufactures use 2 1/2 " blocks with the remainder of the chamber and the forcing cone in the barrel- the reasoning is that that is no different than a pump or autolader where the entire chamber is in the "barrel". the difference being is the auto/ pump is usually on continuous piece of steel where the o/u is two pieces(usually) however with some o/u or s/s there is no monoblock, just two barrels soldered to a spacer. That setup could probably be converted safely, provided there is enough steel in the chamber walls that far up the barrel.

Here in the USA 3 1/2" was introduced( as was the 10 gage)because we had to use less effective steel shot.If you already have a 3" the gain going to 3 1/2" wouldn't be all that great. But whether it could be done I would think it would depend on the dimensions of the particular gun.

Just curious , is it possible or practicle to ream out side by side or O/U shotguns chambered in 3" magnum to the 3 1/2" super magnum?