63 Dart rear axle relocation.

Tin, there are a couple of ways to work around what you'd like to do.
In order to use an 18x10 wheel, you'll probably need a narrowed rear end, or some very exact measurements and precise offset and backspacing on the wheels.
For tire HEIGHT you can either move the diffy back about an inch, or you can trim the front of the wheel well opening to gain the necessary clearance - or a little of both.
For WIDTH you will probably need to minitub, and either use the offset spring hangar kit, or move the springs into the frame rails. Once this is done, you should probably have your wheels with mounted tire so you can set them in the finished wheelwells and measure from inside wheel to wheel to determine your final rear end width if you're getting it cut.
There are quite a few old threads on these subjects, so if you do a search you'll find lots of how-to posts and photos.
Good luck.