use the 1.5 bottom,when you put the z6 or y8 head on a d15,it raises the compression quiet a bit. the y8 is a better head because the compression ratio (cr) will be slightly higher. check out this d-series compression calulator
http://www.knology.net/~jediklc/
the z6 head + d15 bottom = 10.1-1 cr
the y8 head + d15 bottom = 10.52-1 cr
the z6 head + d15 bottom w/2 layer head gasket=10.86-1 cr
the y8 head + d15 bottom w/2 layer head gasket=10.41-1 cr
the d16y8 stock = 9.6-1 cr
the d16z6 stock = 9.13-1 cr

 

woops i wrote the 2 layer gasket cr backwards