First off i'd like to say if you think you can swap in a 03 cobra motor for 10,000 you are nuts. You'd be lucky to get the motor and trans for that much, and even if you do, the k-member is going to cost a bundle along with a set of headers and h pipe that cost you around 1500.
I've watched them put a 01 4.6 cobra into a notch at my local shop, it's long tedius and Expensive. They are now modding the brakes to get them to work. It looks to be a project that's going to cost the owner over 15,000 by the time it's done.
Don't get me wrong it's sweet project, it's just sooooo expensive in the long run
I'd also go against the 351, i don't see the purpose in it, when a 347 would be lighter, need less specialty parts, and produce about the same HP.
There's a guy down at the shop that has a gt with a 347, N/A with a sportsman block, made 444 to the wheels and ran mid 10's. Motor looks the same as any other modded 5.0.