Wassup man, This pit has been giving you problems from the start right? I haven't gone back to check your other posts but if my memory serves me right (and sometimes it doesn't, so forgive me if I'm wrong) you made that out of compressor tanks joined together which can tend to be on the thin side. If your steel is not thick enough to hold the heat in your pit, with a pit of that length no matter what you do with a firebox on the far end of it you will always have heat fall off. Have you tried insulating the pit somehow? I don't really know what to tell you. Sorry.
1 - 100 lb. self-made cylinder grill/smoker
1 - 100 lb. self-made cylinder smoker
1 - 20 gal. self-made grill/smoker
1 - 25 gal. self-made vertical smoker
1 - 120 gal. self-made trailer stick-burner