Jon,

On the Uboat.net main screen select "U-Boat Types," then select the IXC and the IXC/40 and you can see what was physically different.

Type IXC
Displacement:
(tons) 1120 (sf)
1232 (sm)
1540 (total)
Length: (m) 76,76 oa
58,75 ph
Beam: (m) 6,76 oa
4,40 ph
Draught: (draft) 4,70 m
Height: 9,40 m

Type IXC/40
Displacement:
(tons) 1120 (sf)
1232 (sm)
1545 (total)
Length: (m) 76,76 oa
58,75 ph
Beam: (m) 6,86 oa
4,44 ph
Draught: (draft) 4,67 m
Height: 9,60 m

My personal opinion on a 1/144 scale model you are just splitting hairs....

Regards,
Don_

"A man's got to know his limitations..."
Harry Callahan, SFPD