It's not how tall you ride in the saddle, it's the fancy way you get on and off.
If a stroked 302 in a Cobra is underpowered what is a Corvette Z06 with only a 405HP 350?
I run a soccer mom motor(302 out of a Mountaineer) in my Cobra and it run with most big blocks up to about 120 MPH. I run 3.73:1 gears so it is real quick and who needs a lot of top speed capability in a Cobra? Plus it is kind of fun to open up the hood and show the guy he just got beat by a 302.
One of the fastest cobra replicas is quick1's car and he is running a 289! If I remember correctly the Dragon Snakes were 289 motors.
A big block is real powerful but also harder to harness all the torque. IMHO a cobra needs very little torque bot it does need HP and 350 hp is about all you will really need unless you plan on racing every day.
Most people lump 351 engines in with 302 engines when resale is considered. Yes you get more for selling a big block but it does cost more up front.
Remember it is your car so do what you want not what we say you should do.