does it take a while to over heat, or does it do it immediately? You said "a short drive" 1 block, 10 miles, what. Does it do it at highway speeds, or only in town. I think your problem is you don't have all the right ducting and shrouding, (see I told you you did more than just remove the A/C) This is the first time you mentioned it wasn't actually a 77 chevy any more. Is the fan and radiator in the same spot? is all your air going around the radiator and not through it? Go back in the posts about 5 or 6 pages and the post is something like "engine boiling over" it covers just about every possibility. So we don't have to run it through the mill again.
