how hard is it to modify the tunnel? will i need to get that certed?

I think you'll find that the S13 SR20 and S13 CA18 crossmembers are the same. So using this information, if you use engine and engine mounts that come with either engine, that it will sit on the same crossmember. But the SR20 won't fit in some 1200 installs because of brake/clutch master cylinder clearances.
Also the SR20 gearbox bellhousing is ~1 inch larger in diameter, so you'd need to modify the 1200 tunnel to make it fit.

i tried to email you but it failed. do you make fitting kits for sr20det into a 1200? do i just use the sr20 gearbox? how much these kits cost? email
scotty_ _29@hotmail.com

My email is andrew@winsoft.net.au
Im pretty sure airmail should be allrigth Ive been sending most through the normal post it cost around $15 Aus to post interstate probably around $20-$30 to NZ I can make some inquiries if you want I have been doing them exchange but since your overseas Ill have to get one from a wrecker to modify if you want one.
Cheers Andrew

you got ca18detmans email? im from new zealand does he ship internationally?

depending on the km's the engine has done it could be as low as $500 for a very used one, or up to $2000 for a low km engine. Gearboxes, if you want a quick easy fit then the MR30 "Stumpy" Skyline box is best, but otherwise with slight mods the S13 5speed manual goes in, the S13 box is the stronger one.

Talk to Andrew Denniss (CA18DET_Man) he does crossmembers to suit the CA18 into 1200's. He does a perfect job for only a faction of what maddat charge.

Cheers
Damo

Hey mate im doing the same conversion and got a fitting kit through maddat just give him a call and get him to send you a kit cod

hey im wanting to put a ca18det into my 1200 ute. what sort of price range should i be looking to pay for a engine? what gearbox should i use? anyone know where i can get a engine fitting kit for this(i tried maddat.com.au but he hasnt yet emailed me back). cheers