first off, its a header, not headers

those are studs in the cat, (at least on the 4g), not bolts

the ends of the 3 cat studs are not threaded
they have a tapered end for aligning the exhaust pipe

i believe the nuts are 14mm and they are also rarely easy to get off
especially if they are from the factory

what you are gonna want to do is use alot of wd40, and a jimmi bar
so what the hell is a jimmi bar
well its a pipe of variable length and size tailored to suit your needs based on space requirements and torque requirments
i have several of them, different lengths and sizes for tight areas or all out mayhem

as far as the 3 bolts that connect the 2 pieces of the header, you can either drop the front crossmember by removing the front engine mount, radius rods, and the front/rear cross stiffener, or you can put three 6 inch entensions on the ratchet and deal with it.

note: it is possible to snap exhaust bolts