As others have replied, 451 and 406 are not interchangeable. See
Sheldon Brown's web page on tire size designations for more details. In short, 406 is also known as 20"x1.5" to 20"x2.125" (decimal inch sizing), while 451 is also known as 20"x1-1/8", 20"x1-1/4" or 20"x1-3/8" (fractional inch sizing).
There are inner tubes sold as being compatible with both 406 and 451 (e.g., Continental), but I find them much more difficult to use with 451 rims and tires. They are smaller in diameter and so harder to mount on the rim, and they also make it harder to seat the tire bead on the rim.
As for more alternatives to Stelvios, there's the Schwalbe Ultremo ZX (451x23mm, 160 psi), but it's very expensive (street price over $75, last time I Googled). I've used Duranos and Panaracer Minits Lites satisfactorily, but I don't do loaded touring.