Not too bad, you have the right idea. You need to have your lines flow into each other smoother. You want to have as equally spaced polygons as possible. Car/Truck have more curves than you realize, even if they are a fairly boxy vehicle. You should always be thinking "How can I optimize and reduce the poly count while keeping a good aesthetics?". You can get away with a fairly low poly count while keeping a high quality look due to varying degrees of edge split/phong(typically 40-80 degrees). Be aware that 1960-70's Nissan/Datsun have a ton of compound curves which are very difficult to model for a novice. This small part of the door of this Datsun 620 I have been modeling for the past couple years took me an estimated 50 hours(or more) and many attempts to get it right. So if feel like something doesn't look right you always try to salvage the mesh or start fresh on that area(sometimes starting new can help you clear your mind of what you were doing wrong). I would also recommend modeling off of a base model C10 Skyline and then adding the GTR bits later. You should also do a ton of research and save as many pictures of obscure angles, detail shots, ect. to your pc so you can quickly compare it to your model. To give you a rough idea of what your poly flow should somewhat look like I have done a quick edit showing you how lines in different area should somewhat look like.