I'm working with a similar conversion though mine goes other way around. Here is a short summary of the differences:
- there is two different flavours of the front mount. The one found in XR2i 16V and the one in Si. The Si one has an additional support which bolts onto the inner front wing. The bolt pattern in the chassisrail is not compatible wirth the latter type in pre -92 chassis.
- the rear engine mount mounting place is in different location. You can just drill an extra hole into the 8V version and off you go.
- The bulkhead is quite different. The cutout and seal for the steering column is in different location and different shape. The steering racks mounting points are in different place because mk3,5 chassis have more clearance and strenghtened mounting points compeared to mk3 one. This is required to house the PAS setup.
- lower ball joint, suspension arm, outer CV, hub, bearings and strut are all different desing and none of those are interchangeable individually with the corresponding mk3 items. You can change the suspension as a whole though.

There is nothing major, but lot of details to take account when fitting 16V engine into 8V car or other way around.