I have done the ca conversion into a r31 pintara(the purple pinny) it works well.
even positioned like the old ca20e motor .
ive also done a sr20 conversion into a r31 pintara ....... not once but 2 times now 1 has a s14 sr20de motor in a pinny with the std crossmember and redrilled holes 10mm forward or back i cannot remember .
and the other is a s13 sr20det red top into a pinny using the s13 crossmember modifyed to suit a r31 ,then a s13 steering rack, hr31 rack rods for the kyb rack ,hr31 front struts
i cannot see why the ca s13 crossmember wont work and make the ca18 sit as far back as possiable , but same deal aplies your gonna need s13 rack hr31 rack rods to suit the kyb rack and then hr struts or you could go s13 rack rods and s13 struts etc
if ya need any more info just pm me