For a 450-400 NE the weight can kept in the 9 1/2 to 10 pound range and still be comfortable to shoot.
Many english doubles are quite a bit heavier than that.

For your thickness at the muzzels just work on approx 100 thou wall which is about the norm for many doubles.
Ideal barrel length 26 inches.
Flats to top of barrels at face 1.170" min.
Barrels remain full dia for approx the length of the chambers then fast taper over short length and then gradual taper all the way to muzzels.
This type of profile leaves plenty of weight where it is needed to achieve nice balance and handling.
Striker spacing approx 1.040".
Width of barrels at face 2.150".

Just a few thoughts based on one of the nicest handling doubles I've ever used.