Can answer your questions only thing is a have a ca18det in my hachi. if you want to use your original ae86 cluster you will have to get yourself a rb20 speedo cable and just change the end of it to suit your cluster.
You should not have a issue with engineering it as long as everything is done correctly eg. engine mounts, gbox xmember, moded tunnel. brakes up graded, fuel system under the car etc.
PM me if you have any questions otherwise GL