Grubby John the numbers you listed 4.25 and 3.98 are 452 cubes. I know I built one. Any way, Could you fill in a couple of questions, What trans are you going with?? If its a wide ratio one and a 1-1 5th gear with a high 1st gear, get a 3.07 rearend. If you are going with a overdrive trains then go with the 3.31 gears. If the first gear ratio in the trans is higher that 3.00 you will never use it unless you are in a parade. I have 2 trannys. A Richmond 6 spd with a 3.25 first gear and never use it. I have a G-Force race trans with a 3.15 and have this trans going back to G-Force for a gear change to a 2.87 with the 3.31 gears. I also have a 3.07 and 2.88 jag centered for my car. Both need to be beefed up for the extra Torque and HP my 482 makes. I liked my 452, runs on reg gas and make 368 hp and 448 torque before I did some extras to the motor. The limiting factor in your car is the HP and Torque rating of the parts you install and HOW MUCH ABUSE you are going to give the car. It takes even the best drivers a couple of hours behind the wheel to get used to a cobra.
It's a whole different animal.
. Treat it like a loaded gun or a 800 ape, you will have no problems. Get too smart for your own good and there are others on here that will tell you about how fast this car will bite you
You have some good info from Keith and Barry on motors. My only differ from these PRO builders is, I build torque motors for my car, not HP. My motors(2) FE's both make more torque than HP. I am building a 498 with more torque, 58mm TWM setup and a KCR custom cam hydro roller. Keith said easy 600-600 HP and torque. All the power will be in before 6,200 rpm. My limit for my stroker is 6,500 rpms. I don't like to high rev these motors, even with the BBC rods in them. 3.54 gearing IMO is for a more nasty motor that doesn't come into the torque band until 2,800 rpm or higher. You don't want to lug this kind of motor at highway speeds. You will burn alot more gas also. Ask Excaliper, (Ernie) on the furom about his 2 motors he has had for his cobra and about the less being better than more. Compression, and the biggie camshafts. He found out in his case that a tamer motor was only .2 slower than the street motor he now has. His gas milage also doubled. What every you end up with, over build the car for safety and abuse. Rick L