This has come up a few times now and as decent shells 1.8 & 2.0 become more scarce using lower spec cars as a project base makes more sense.

The main difference between the 1.3-1.6 shell and the 1.8-2.0 shell is that the 1.8-2.0 has a more beefy front subframe and crossmember setup.

The issue is that you cannot simply transfer the subframe across. The longer subframe is located by 2 bolts in the centre while the shorter ones have one bolt and a locating dowl.

Longer subframe

Shorter subframe

All is not lost though. The rear bolts line up and the dimensions of the subframes are basically the same otherwise so you can cut the the section out of the shorter arms and weld them into the longer ones. 

Take careful measurements of where everything should be then cut out the section in question from the shorter arms

Which will leave you with this.

Now once you have checked your measurements cut the corresponding section out of the longer arms and transfer the single bolt setup across. I suggest tacking it up then checking it on the car to make sure it lines up with the 2 rear bolts.

And you end up with this

Then there is the front crossmember / radiator support.

It can be cut out of a 1.8 or 2.0 (if you can get a  decent one) and welded into place in the 1.3/1.6. It's basically a straight swap. With the lower arms in place you can bolt it up before welding to make sure it's in the right place.

If you're fitting an intercooler (as I was) you may benefit from making up a one off crossmember to give you extra space. What way you go about that will depend on your needs.

Personally I'd say this mod is only actually needed if you're running big numbers or are planning track days. If you only plan on fitting an XE and are a bit daunted by the task you'll be fine without.