The following link references a study of oblique inlet shape for heat exchangers that might be a good starting point for radiator and oil cooler duct construction. The coordinates below can be scaled for the core size to be used.
X Z
0 1
0.1 0.89
0.2 0.76
0.3 0.63
0.4 0.51
0.5 0.39
0.6 0.29
0.7 0.2
0.8 0.12
0.9 0.06
1 0
Steve Boese
RV6A, 1986 13B NA, RD1A, EC2