I bought the APB pump from SubTech a while ago for my Ohio, as I'm (also) building the entire inside stuff my self from scratch. The APB pump is a little more than 2" long, and app. the same from stud to stud... Allthough it's a fine pump, it sounds like it's a bit too large for you..
Yours Sincerely, Robert Holsting, Denmark
1/81 SSBN Ohio Class scratch builder, more at www.robse.dk
"Never be afraid to try something new; remember that it was amateurs who build Noah's Ark, and professionals who build the Titanic"