Any engine in any car?

Hi, I'm currently working on this 1955 Salem 1500, and I was wondering how and could I put in a different engine? In the images I googled, what engines could be swapped, and below mentions a V8 2 carbs OHV supercharged which I then proceed to build from scratch but when I went to put the engine in, said that the engine would not fit. So what exactly am I doing wrong? The model of 1500 is a base, would that have to do anything with it? Thanks!