The silvia radiator will fit.... I've got one in my ute.
It's all about how you mount it.

I presume you've cut out all the original radiator mounting stuff from the grill area....

Then you just drill a couple of (16mm from memory) holes in the box section looking cross brace in the front of the engine bay.

This will allow the silvia radiator with the standard rubber bushings to slot in..... Sits just right.