[blykins]

You gotta watch about how much hp you put through a factory block. The factory 352, 390, and 428 blocks are known to split between the cam tunnel and mains with sustained hp and rpm. There are many, many variables about where that line of no return is, but personally, I would not build a 600 hp FE with a factory block. If I did, I would convert to cross-bolted ProGram billet main caps and do some other things to the block....and then you're adding a couple grand into the price....and you still have a 45 year old block.

Also, with a factory 352 or 390 block, you're basically looking at a max of 460 cubes or so. With a 428 block, you're looking at a max of 475 cubes. Both are those are with 4.375" stroke cranks. As soon as you switch to an aftermarket block, you're looking at maximum displacements of 510-530 cubes depending on the block.

On the hp thing, it would vary from dyno to dyno. Not insinuating anything here, not making any claims, but you will often see 5-6% (and higher) differences between dynos, based on the dyno brand, who's running it, where they have their weather station in relation to their makeup air, etc. So the point there is to not get hung up on numbers.

It is very hard to tell the difference between 20-30 hp in a Cobra as they are hand-fulls at WOT. It might be easier if you started with 300 and added 20, but if you start with 600 and add 20, you're not gonna be able to tell the difference.

My advice is that if you have the money, either go with a factory 427 S/O block and build it to be a 427, or buy a Pond block and go with a 482/496/510.