Hey Bubble_city,
You have an idea in your mind of what you want, you just have to figure out how to get there. Research all your options and evaluate your skills. There are alot of different ways to create what you want. I have put a 51 GMC on a 1978 C30 frame which is the same as your 77. If you look in my signature below there are links to photos. I have also seen what 641workmaster is talking about by one who cut a portion of the frame out of the old truck to scab onto the newer frame. The original truck frame has a nearly straight topline which is no-where close to the 77 or 78 frame. There would have to be a fairly subtainial cab mount the give the correct placement. The P30 frame is closer to the same topline as your original frame if I remember correctly. The easiest and fastest way to enjoy your truck if you are building it youself is to leave it on the original frame and modify as necessary. Try your best to keep it running and operational with only minor out of commision sessions. Staying motivated on a build is usually difficult for alot of people. If you truck runs now then I would not swap the frame.
Oh, by the way, you need to have some photos for all to review. You can post these on an external photo storage site and link to your post.
