If what you want is 400 hp, do the 302. 400 is not a stretch here, and easy to reach with a very modest budget. Yes, lo-end torque will not be mountain-motor like, but in a Cobra all that power is quite literally useless until you're north of 60 or 70 mph and in a higher gear. Then the sky is the limit....but then, that's not what you are looking for to begin with.
The 331 is, despite what you read here, a far, far better piece than the 347. Yes, yes, we've all heard the love stories for both, more cubic inches, more torque, blah, blah, blah. Not BS, but the difference between them is not discernable in a street car. The difference in either IS noticible over the 302 - but again, we're talking about higher speeds where it's most noticeable. The 331, with the 5.400" rod (the 302 has 5.090"), has the SAME rod ratio as the 302 - which is
not a monumental, make-it-or-break-it proposition to begin with. Despite what you read here, the piston pin does NOT intrude on the bottom ring in a 331; and doesn't in some 347's. Durability/reliability/
oil consumption in a 331 is
no different than the 302, and you might give up - maybe - 20 hp over the 347. Maybe. They'll spin to 7000 all day long without issue, make 430 hp without strain, idle at 800 rpm with an aluminum flywheel, and are - like the 302 - a turn-the-key-and-go motor.
351's do take you into a different realm - 408's/418's/427's do make more power than the 302-based motors, and despite what you read here, they also cost more. There will be more torque down low, which makes for those impressive burnouts. They also go like stink in high gear, which is even more impressive. Dollar for dollar, they're not really much more than a 302-based stroker, and will provide you with power you're not likely to get bored with any time soon. They may be a bit more than what you want, power-wise, to begin with. But - better to have it and not want it, than want it and not have it.
You are assured you won't be bored with any of them. The 302 is a truly fine motor; the 331 makes it all that much more fine. 347's are no less fine, and the pinnacle for 302-based strokers. 351's of all displacements make for plenty of excitement, and in stroked versions can make some truly awesome power. Take your pick - none are wrong. Only thing you really need to decide is how much do I want, and how much do I want to spend.