@CodeFantastic I took a look at your imgur images. By the looks of things you did an automated UV mapping. We modelers usually do it manually, there's an option in blender to mark edges as seams (Ctrl + E / Mark Seam). Marked edges will turn light blue and during unwrapping the UV ( U / Unwrap) Blender will make separate UV islands for faces surrounded by seams. Basically seams are where Blender will cut the UV island from the rest.
After unwrapping we usually merge all similar islands on the map to save space and reuse the texture where possible. Better space management on the texture use the resolution to its fullest. In your texture your tank sides use only 1/4 of the texture space, but they're the most prominent element of the tank, having it use more space will let you add more details and the resolution of those faces will be better in the end.
In this image I marked a window frame with seams so that it unwraps as a separate UV island:
There are some very good tutorials on youtube about blender UV unwrapping techniques.
As for using the UV map as an overlay in your graphic software, make sure that you place it with pixel precision, matching all 4 corners of the map with the 4 corners of your canvas. When I'm working in Photoshop and I drag and drop a UV map onto my canvas - it doesn't always align perfectly, I zoom in and do it manually.