fitting exhaust

Hi Guys
looking for some advice on re-fitting a standard exhaust, I have just changed the collectors and the header pipes and it seems to be blowing from number 1 and 6 where they attach to the engine, is there a sequence that it is best to tighten them up, I basically left everything loose until the front pipes seemed to be located and then gradually tightened everything up but I am guessing the 2 that appear to be blowing have not located flat,so does that mean I have to loosen everything off and start again
thanks
Paul

Paul:

Did you install new copper header gaskets?

NO, I MUST ADMIT I WAS THINKING THAT MAY BE THE CAUSE,EVEN WITH THE OLD WASHERS IN I CAN ONLY JUST GET THE NUTS STARTED ON THE EXHAUST STUD SO WHEN I PUT IN NEW COPPER GASKETS IT WILL BE MORE OF A NIGHTMARE, THANKS FOR YOUR HELP
PAUL

did you put the correct collars in number 1 and 6, I found that the ones on mine are a tad shorter than the ones for the other cylinders