Right tools for the job...

I have been successful reaching all nuts holding manifold with 3/8” drive socket & various extensions with Clifford headers & intake combo. Torqueing top center nut use a universal joint between socket and extension keeping this combination straight as possible. To start those hard to reach lower row studs, use a bit of masking tape in socket to hold nut long enough to get it started.

New studs, and nuts make this job much, much easier, but if new studs ain’t in the cards, clean old stud threads with wire brush until nut ‘threads on’ by hand, than apply never-seize and use new brass nuts.