b16 and b18c5 heads are almost the same as far as castings are concerned and should yet the same size combustion chambers, yielding near the same cr. in contrast, b18c1, aka 1.8l GSRs are a bit different, and the differences are unknown to me. but from what i hear, head gaskets play a small role in bumping up cr as many companies make two layer head gaskets, whereas the stock head gasket is three ply. the biggest factor in achieving a higher cr is the type of pistons you use. so, to get a 12.1 cr like you want, changing out the ls pistons for some b-series vtec pistons is a must. now as far as clearance issues with different pistons, there shouldn't be any. the same is true for higher lift cams.