The Roush is a great motor, but you are paying them a huge profit margin for the name IMO if you add up the sum of the parts, many competent builders can build you the same motor for less. There is merit in the use of the stronger Motorsports/world products or Dart blocks but then again, it's all in how you plan to run the car when it's done. A precision machined stock block351W based 427 is perfectly reliable up to around 525 hp if you use Top quality components and a stud girdle, the biggest hinderance to the 4.17 stroke 351W's is not the rod ratio, it's the huge main bearings that don't like sustained high RPM. Stock block motors just need to be built for a more moderate peak 500 lb/ft torque/500+hp rpm range, a perfect balance is ~4500rpm tq/5500rpm hp peaks with a 6000-6200 rpm redline...with modern aftermarket heads flowing in the 280 @.500" lift this power/tq level can easily be attained with a mild street roller in the ~236 @.050 range. This would not be an all out drag motor, but it will easily propel a 2500 lb Cobra into the high 10's/low 11's if properly set up and in the right hands...quite respectable by any measure regardless of cost. With the relatively low torque peak rpm, the stresses on the bottom end above the torque peak will not be excessive on a well machined motor. Sure it's not optimal, but it's perfectly servicable for the purpose.
As I said, it's all in how you plan to run the car, if you are an avid road racer or drag racer, or if you plan to spray the motor, by all means build an aftermarket block motor. But if you are more a week-end enthusiast who is just looking for that big block grunt and massive mid range torque, a perfectly strong 525 horse stock block 351W-427 can be built for much less. I'm a 'bang for the buck' approch type of guy, I always build with my head first and not with my wallet, I don;t know, I've always been that way. At first it was because as a kid with no money, IT HAD TO BE THAT WAY, now, I have the choice, but I still prefer to spend less and put the difference somewhere else that makes more sense to me, like kids College funds, investments and charities. Cobra builders tend to be obviously a high-end demographic, so just like at Barrett-Jackson, many people tend to brag about how MUCH they spent on their cars instead of how little, so in the end it's all in what you are looking for.
my .02
Wize
www.streetwizeperformance.com