Let me get this right... you're drilling out the quill to allow a draw bar through to pull the ER collet chuck into the quill... so what are you planning to tap? did I miss something?
I decided against drilling all the way through the quill as I could only get a 6mm hole through the splined section and the chuck needs M10, resulting in a long thin drawbar and some sort of thresd adaptor.

Bill suggested drilling from the chuck end up into the quill and then use a bolt through the chuck to secure it (similar to some drill chucks)

So i need to to drill a 7.5mm hole in the quill and then tap it for M8, dose that make sense?