There are heaps of threads on how to make a 27 from 25/26 parts, you use a RB26 crank, rods and pistons with an overbore of .88mm or 1mm i cant remember. or you can buy the 2.7 stroker kit
mine has been done using forged oversized 26 pistons rods ect had the head bored, ect also has a thicker head gasket ( i'm not sure on thickness)
The motor build cost around 10k so it was done properly
edit: one of the few threads on here about them http://www.skylinesa...27-info-needed/
http://www.skylinesaustralia.com/forums/topic/321488-rb26-crank-in-a-rb25/