seat belt mounting points

I am planning on using 4 point mountings ,threepoints are obvious on the car ,where have people placed the mounting hole for the front outer strap (lap strap) ?along side the front outer seat mount hole??

You mean anit-sub strap or crotch strap? Drill it into the metal support that goes across the floor in the middle. Use load bearing plates to make sure it doesn't pull through, but to be honest if you ever got into bad enough of a wreck to even begin to pull one of those through you won't be making it! The anti sub never sees much force and if it does you're done.