You don't need to modify the cross member like in the picture above, also it's much easier and neater to make the mountings from plate. Have a look at all the pics under my profile, I did the same conversion, just with a 16V, but the mounting and positioning of the motor will be exactly the same, as will most of the water pipes and stuff...

You don't need to modify the tunnel to fit the 21R gearbox, apart from a bit of panel beating with a ball peen hammer and moving the lever hole backwards about 100mm.