Hey, Lucien! I know this isn't exactly what you're looking for, but it might get you on the right path...
I don't know if anyone makes aftermarket kits of just the turrets in 1/72 but i think swanny converted a B-24 (or was it a PB4Y?) into one with a ballturret in the nose position (I think). you might shoot him an e-mail to ask. Also, one of the first questions i asked in this forum is if anyone made aftermarket parts for 1/72 50 Cal guns... here's what i the guys/girls here gave me:
http://www.kitreview.com/reviews/karayabarrelsreviewbg_1.htm
Shame they don't make it in 1/72... yet.
Hope this helped!