how do you know were top dead center is ? this is very hard to check when it s together i think you have something not rigth .start with one thing at a time is the fireing order rigth ? check it. is the lifters set rigth check them . do you have a intake leak? all of this or one thing can give you .what you have who put the engine to gether ? is the cam in it rigth ? what is the duration@ 0.50 what cam you have? did you try any thing that i reply to you on? i have many built many big blocks with 275 to280 duration @ 0.50 and run them on the street with out to many problems here is somethings to think about you said it will not idle below 1000 check you many not be on your idle circuit you may have to crack the rear barrels so you can set idle mix s