can someone please tell me how i bolt a ss bigmouth snorkel to my ef?

Probably

ur a smart a*** sam lol

Sorry man couldnt help myself...
What are you having trouble with? The snorkel itself or the airbox mount side?

getting it to sit properly is modifying the box so it plugs in hard?

I have no experience with fitting these things, but from what i have read around people have needed to some of the following:
- Re-drill snorkel mounts (ie: move the front of the snorkel further towards the front of the car). This is due to some possible clearance issues with pulley and the belt
- Cut the airbox to the new inlet size. (This will help it sit nicely)
- Massage the snorkel to help clear pulley and fans.

See if any of these help at all

I had no problem with clearances or having to re-drill snorkel mounting holes on my EF XR6.

As the mouth of the snorkel is bigger (go figure!), the rubber seal on the underside of the bonnet no longer seals to it and lets in hot air. So I drilled two holes a few cms f@rt back, stretched the seal and plugged it into the new holes. Now it seals, and doesn't let in any hot engine compartment air.

The only significant modification is cutting a new hole into the airbox. The SS Inductions piece is about twice the size of the stocker. I didn't have a jigsaw or similar, so I spent a hour or so with a drill, utility knife and round grinding stone getting the hole to the right size so the new piece would fit perfectly. You will need to trim back the external ribs on the airbox to allow the SS part to sit flat, but it's pretty straightforward. Just use the new part as a template. To be on the safe side, make the first cut a little smaller than you expect to finish up. It's much easier to make the hole bigger than it is to make it smaller But it wasn't a major at all - in fact, I quite enjoyed the job. And the difference was worth it. It does make a lot more induction noise though.

There's some info here that might help you:
ford-4l-and-6-cylinder-f1/aunger-bigmouth-vs-ss-inductions-bigmouth-cai-t90842-15.html

The only other thing to note is the SS Inductions Big Mouth and the airbox must always be installed and removed together as a unit. The snorkel is so big and extends into (not over) the airbox entry, so it cannot be removed or inserted with the airbox still bolted to the car. Getting the front airbox bolt in can be a bit fussy with the snorkel in place.