Another Dumb Question (273? 318?)

I pulled the heads in my 69 Dart which I believed was all original. When I first got the car I checked the VIN, 273 LA engine but when I pulled the valve covers I found 318 heads. Now I'm confused, is my block a 318 too? Was it common for them to put 318 heads on the 273 or did someone swap heads in the past? Its a low miles car, 54k when I got it. I was told then that it had sat in a garage for 30 years, coming from a family friend i believe them. Any help is good help, thanks

-TheOriginalDart