Calling what was done to the barrels of your Lefever "monblocking" is inaccurate.
A monoblock is a single piece of steel, usually a forging, that is then machined to accept barrels. The components left after the barrels are sawed off of a gun for "sleeving" are of multiple pieces, under rib, upper rib, tube, fore end loop, etc, and cannot be considered a monoblock.
It is a completely incorrect use of the term.
I have a great deal of respect for 12 gauge, 3 1/2" magnum loads. I find them to be unpleasant to shoot, even in autoloading shotguns designed to fire them. I understand most barrel failures are due to a restriction, but, I would be very hesitant to use those loads in a gun not specifically designed for them.
I think the pictured gun might work for many years with low pressure loads being used in it. That said, I'm not going to buy it to find out.
There exist others without the modification (and screwy lettering on the barrels) that would be better using examples.
Best,
Ted
