> 3 For the water radiator I'm planning of using a core 3 times the size of
> the oil cooler. Does anybody think that it will be too small?

If you are talking core surface area the oil cooler is approx 19x4 = 76 sq inch.  so 3 times that would be 228 sq inch.  Two
of the GM evaporator cores will give you 95x2 = 190 sq inches so the area sounds OK.  How about the core volumn? Yes, thickness does play a part in cooling.  The two gm cores have a core volumn of 2x( 9x10x3.5) = 630 cubic inches. Just for a point of reference.

Ed,
The oil cooler has 2” thick core so, 76 sq inches x 2 =152 cubic inches.  Rad. will be 152 x 3=458cubic inches.
Bulent