the engine itself is pretty much unchanged... same motor mount and same engine to tranny bolt pattern, all the ca18 bolts in the s12 or s13, which ever version it is. The only difference is between both versions lies in the transmission bolted to it : the s12 version uses the short FS5W71C transmission, the same one as in the mkII cars so the shifter fit in the same location as stock... the s13 version uses the same transmission, but in a longer form (like the one on the sr20det and ka24) and push the shifter backwards, so you need to cut your transmission tunnel a little, although both trannies share the same internals.
If you have a mkII car, use your current transmission on any ca18det block and it's a direct bolt-on on the engine mounts and transmission mounts, if you use the s13 transmission, you need to mod the tranny mount and cut the transmission tunnel.
If you have a mkI car, you can still use your stock tranny, but it's will not withstand the same power level that the C will...