$400 each!!!
You are being ripped off.
$250 each, mounted and balanced is the most I'd pay anywhere in the US. Try a different shop.
But yes, you can replace the FRONT TWO with the 225/70R19.5...dunno about the 187's. Move the fronts to the rear and put the wider tires on the front. The problem is the width. You'd want to find a tire of the same width so the flexing as you're driving won't make the tires rub together on the dulies. Which is why you'd usually put them on the front.
PS You can't tell if the dualies will rub together just sitting on the pavement. It's when you're rolling and hit a bump that compression makes the sidewalls pooch out that they rub together.
Edit: Changed the 75R to the correct value used on my rig, 70R. Mistyped originally.