Ben,
  Your trepidation is well warranted. The archives are filled with tales of woe
about broken off exhaust studs. Since you were able to partly back out a stud I
would then soak it with a penetrating oil and screw it back in. Gently repeat
until it is out.
  Find a parts store, preferably that has a lot of air cooled VW stuff, and
purchase some new studs and 12mm _copper_ exhaust nuts. The air cooled
engines use the same size as the WBX. These are easier to remove the next time
since they do not rust. Also pick up a tube of anti-seize compound. Home Depot
also carries some metric hardware. As a last resort you may have to buy bolts
and cut off the heads.
  I do not recommend using bolts here simply because you do not know if it is
threaded all the way in. With studs you know  you are using all the threads (in
the head) since it bottoms out. In the past I used a bolt, only to find it
missing, along with half the threads in the head!
  Also pick up an installation kit for oil gauges plus metric adapters. This kit
contains the tiny nylon tubing. I added some protection by sliding pieces of
fuel line over it in ares that might chafe. I mounted the sender near the coil
using two existing holes and a "U" clamp used in TV antenna work.
  I retained the original low pressure switch by adding a tee. There are some
picture at http://neksiwel.20m.com/ .

Good Luck
Ken Lewis