Seal the door when done, this mod has been done thousands of times without issue. There are many cars that come from the factory with trap doors for the fuel pump.
here is a write up on a car with the older metal tank, should be the same as your friends.
http://www.taekwondoplus.org/z28/fuelpump.html
this place has pumps cheap
http://www.expressfuelpumps.com/