Block should not matter - Deck Height does. The Piston-to-Deck clearance on a B16 is .020", and .030" on a B18 series. So the piston has .010 extra clearance to the valves on a B18.

Where this is offset, is when you look at the compression height - The CH of a stock piston is 1.183 for a GSR, and 1.210 for a PCT.

So, then, what's the difference in lift between the CTR cams, and the GSR cams... If it's .0170 or more, and the GSR cam has less lift, then it should work fine and have stock clearance.
If you're running aftermarket cams, which have much higher lifts and durations, then maybe not!