Depending on your skills & understanding I might consider paying a pro to do it.  It's a lot cheaper and faster than screwing it up yourself first then paying someone to do it right.

Having said that, you can also O-ring the cylinder instead of the head. Not sure if one is better or worse than the other.  Also there is a very precise angle you'll need to cut the o-ring groove so that it pinches the O-ring properly and completes the seal.  If not done properly it will leak under load.  Again consult a pro 

On the restricting of the head yes there is a reason.  The restriction backs up the coolant flow and pushes it back wards over the rear of the cylinder to help cool the intake side move evenly due to the tilt of the engine.