Ummm... no.
I'm not saying Stewart isn't a good driver, he's one of the few in NASCAR with any actual talent.... but no, he would not be competitive in f 1. It's doubfull he' make it through a practice session without crashing.
You can't even fathom how different the cars are, and how much faster they are. Jeff Gordon found out, and happens to be one of the other few talented NASCAR drivers... and he looped the car because he wasn't driving fast enough. F1 car performance, in race trim:
-Mid-high 8 second quarter mile
-900hp, 1300lb
-So light that they can't grip the track without downforce, meaning you fly off the track if you're going too slow.
People make fun of the automation of the cars, and they're ignorant morons who have no idea how complicated it actually is:
This is all shit that the driver has to use while driving the fastest racecar on plant earth in wheel-to-wheel competiton.
Competitive F1 drivers prepare their entire carreers to drive those cars, and even the best have rough rookie years. A spot on a competitive F1 team is harder to get than a spot on the space shuttle.
Backin the day, there were a couple of 2-seater cars develpoed, to give people rides. These were snow, had worse handling, and more drag... and when driven at 100%, the passengers would not stop screaming.
So no, tony Stewart would not be competitive in F1, not without half a decade of practice, and that's giving him the benefit of the doubt.