It all just makes zero sense to me. Only thing for sure is that you'll drop a shit ton of money, ruin 2 perfectly good cars and end up with a product that's no where near as good as a 991 and no where near as cool as a 964.
There is a really good reason why more people aren't doing stuff like this. Whilst it may come as a surprise, Porsche make and engineer great cars, typically better than any independent custom fabricator regardless of how much they charge.
Keep it simple.