I just happen to use port #5508 for a Tamron 90mm which extends way further than the Nikon 105. So I've also been able to use the Nikon + T3/4 setup with the longer port.
I just bought a Tamron 90mm SP AF Macro (not the newer DI version), and figure I'll wind up with an Ikelite housing for my D200. I was thinking the shorter one would work, as the Ikelite site says for non-focus ports this one works:
#5505.58 Lens length less than 6.1" (15.5cm)
I measure about 5 3/4" for the Tamron when fully extended, so that seems right. For flat ports with focus, the 5507 gives another centimeter:
#5507...... Lens length less than 6.5" (16.5cm)
#5508...... Lens length less than 7.5" (19cm)
So did you need the extra inch to use the diopters? I realize now I'd need the 5508 if I intend to use a teleconverter... such as Tamron's SP AF1.4X Teleconverter... and that the 43.5mm SP AF2X Teleconverter that I ordered from TheCameraBox (only $139!) probably won't fit even with the 5508!