69 Globetrotter Air Conditioner Looking.

I have a problem with the A/C unit leaking. Upon inspection I realized that while it works it does not fit. A gap between the top of the roof line and the A/C Unit is about 2inches wide. It's been suggested that Spray Foam would correct the gap. I would prefer to fix it right, even if it calls for getting another A/C Unit that would fit my 69 Globetrotter 21'. If anyone on here could tell me what to look for in an A/C Unit that would fit, I would surely appreciate the help.