I am doing a ca18det conversion . Have already installed the engine and have wired up the loom. Just wanted to know if someone knows how to combine the 1200 loom with ca18det loom. What all wires will have to be linked and the which relays will I need.
All help will be appreciated.

You pretty much leave the 1200 loom alone. You can chop off the couple of wires that went on the A12. You use the 1200 loom to trigger the starter motor. That should go straight on (it's a single spade). You can also wire the CA alternator into the standard loom where the original one's fat 12V wire is (I think it's white with red tracer). You need to splice into the datsun loom for the ignition relay trigger. You can, although I don't think it's essential, splice into the cranking 12v signal in the original loom for the signal to the CA computer as it has an input for cranking 12v.

I think that's pretty much it.

I noticed the oil pressure switch too.

Change the plug over so the light will still come on on the dash.

Will I have to add any relays.

It depends on how much of the silvia loom you are using. If you are using the relays and fusible links of the silvia loom then you shouldn't need to add any. If you have pissed that off then you will need to add some.

I just have the EFI loom which is wired to the engine and to the ECU. I guess I will have to work out the relay wiring.
Thanks for your help.