The fact that the compression is low is not as concerning as how much it varies. If you bought this rebuilt I would ask the builder to explain this. Have you put a vacuum gauge on it. If the needle bounces it indicates tight valves. If you are absolutely sure the valves are not to tight it would apear you have a more serious issue. A smog engine should be 110-117 compression. Most were in the 7.5 to 8:1 compressing area so that would put you in the 110-117 at sea level. You have some cylinders in the proper range. I would concentrate on the low ones if you are working on valve adjustment.
once you are sure the lower end is healthy, a set of vortec heads and a good intake would probably make a significant difference.
Do you have casting numbers for the heads
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