It is very straightforward. The rectangular hole for the remote shifter is slightly flanged around the edge to give it more stiffness, but that's also easy.
The only other modification is drilling the 3 holes for the rear mounting bracket and there are two holes just in front of the transverse cross-member and one behind. I believe the bolts are 5/16" UNF.
This is definitely the best way forward with a Mk.1 as it maintains it with period parts. I guess it will be a non-'S' engine, but these days that is accepted for a Mk.1 up-grade, especially if it was a pre-A+ as that modification was often carried out on Mk.1 cars from the time the 1275 non-'S' block was introduced in about 1968 - '69.
If you are stuck on the hole position I could remove the gaiter from mine and take some measurements, but this may take me a few days to do. Let me know if this will help.