I believe you would have to use B18C head studs. The B16A and B18C5 use the same PR3 casting for their heads. ARP one part number for the B16A and one for the B16B/B18C. The B16B uses B16A crank and rods in a B18C block. All this leads me to believe that you should order the B16A studs.

If you want to bump the compression ratio a little bit you can go with a thinner headgasket, otherwise OEM is fine.

All B-series hydraulic transmissions can use the same clutch and flywheel parts.

As for getting your assorted exhaust pieces to fit, I would call up Landspeed Racing. If you tell them which header and exhaust you have (i.e. which application) they should be able to fab you up a Carsound cat in the right length with the proper flanges.

What do you mean about cam gears? The stock ones? The higher deck height of the B18C block compared to the B16A block might put the cam timing off ever so slightly but I can't imagine it'd be much of an issue. If you're just talking about adjustable cam gears then they're the same for all B-series motors.

Be sure to use a B18C5 timing belt.