MFI injectors are screwed in place. After removal, install plugs which you can get from PMO.
However, it sounds like you may have some CIS heads which have the sleeves and friction-fit injectors. You can cut some threads and install a plug or read what others suggest. Sorry I couldn't help more.
Not sure these heads are compatible with your cylinders. Do you have B. Anderson's book to refer to?
Sherwood Lee
http://members.rennlist.org/911pcars