So, it just occurred to me that I could have made that card for my grandmother about a thousand times easier.
I spent forever masking off part of that long rectangle to avoid a dotted line that was part of the stamp--and I realized today that I should have just flipped that stamp over, or any of my other stamps, and used the back. Duh! Here's a good video by Lisa Spangler explaining this! I wish I had remembered this tip! It's just a little hard to do with non-symmetrical or non-basic stamps, but this would have been perfect here!