It's generally accepted that pullers are more efficient, especially when used with a shroud that covers the entire surface of the radiator core. A SPAL is something like 2700 cfms, and they make a slimline version that I have had good luck with.

I would have suggested that you use a SPAL 16 incher, but you only have 15 inches to play with, so instead I think two 10 inch ones , staggered, on a shroud would work well. From what I understand, two 10 inchers do not equal a 20 inch fan in efficiency, but they do move a fair amount of air.

The shroud should be made so that it is about 1/2 inch off of the surface of the fins, so that air can be dragged clear across the core. Either aluminum or sheet metal can be formed to the shape you need, then you can cut the correct sized hole(s) for the fan(s).

Don