It has been a long time since I replaced a manifold but critical points I remember are:
Drain some coolant from the block because there will be coolant right at the top of the heads
Head and block machining over the years can change angles and cause meeting services to not properly meet. Also, a used manifold that has been machined to fit such a situation will not want to fit properly on an original dimensioned block and heads.
Try to lift the manifold firmly straight up. The telltale oil leakage should be apparent if coming from a bad gasket or mating surface.
Throw away what ever end-valley gaskets your manifold gasket kit provides and just use a proper RTV silicone gasket maker. Follow instructions.
Hope this helps.
Larry
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