If I remember correctly I would put the collars on first, 1 bolt on lightly, slip the exhaust under, up and in. Bolt the back on mid way back, then remove each collar and put the 2 x 1/2 in and bolt it on. NOW that was my 1981 cb750 custom. 4 into 4. It may not work with a header. do not forget to put the gaskets in first.
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Get 3 other people to help you
Hope this helps