A 45" door is quite unusual, but then, an 8" marble threshold is a bit unusual also.
There are many sources for thresholds, plain and fancy.
Here is one:
What is the door material? Is the floor out of level? Can the threshold be run with the floor and the door be custom cut to work with the threshold? How tall does the threshold need to be to solve the problem - will it become a stumbling block?
Here are some solutions: Set the threshold and cut the door to match Get a threshold that matches the gap at its narrowest point and shim the rest to door/floor grade. Well set thresholds are grout packed anyway, so setting the threshold in wet mortar allows custom setting the threshold. The edges can be caulked once the grout has set. There are automatic door bottoms that raise and lower. The amount of travel is limited, but they are made to accommodate both uneven gap and the need to clear flooring. When the door is closed, a pin hits the jamb and pushes the weatherstrip down to seal to the threshold.: