JMO- If it's your first attempt at something like this, you may be Flirtin' with Disaster (forgive the Molly Hatchett reference). I would at the very least pull the dash pad. It's a pain in the patoot, but it could save you days of banging your head on concrete walls while spewing out four letter words you didn't even realize you knew... The worst has a tendency of happening when you try to take the easy way out. Dressing/finishing the weld on the interior will also be almost impossible, but that area will be covered by the A pillar trim. Welding spatter tends to go places you never planned, and have effects you didn't foresee. I'm sure BP has it down to a science, he's a pro- but I wouldn't trust myself on this, and it's not my first time at the dance. Even the windshield, no matter how well you think you have it protected, if you get even a little spatter that finds it's way through, you'll be staring at it every time you get behind the wheel. Re-commence head banging. I once was welding (carefully, so I thought) a c-pillar seam and blew out a rear window- wasn't even near it... probably a freak occurrence, but why tempt fate?
Ultimately, it's your call. Feel lucky?
