I've done the BBF stuff with the Boss 9 heads, etc. The aftermarket blocks needed to get to the 598-604 ci range are Ford Racing blocks with siamese bores and I don't think they are physically larger than a production block, but they pack a 4.500-4.600" bore size vs the 4.360" factory bore. With that being said, the heads could rain on your parade as they are huge. I think Jay with Vintage says that they fit in a Backdraft, but I haven't heard anyone say anything about a Contemporary.
The P51 heads will make as much power as the Boss 9 heads at a tremendously lower cost. The Boss 9 heads aren't expensive by themselves, but it's the $1500 rocker arms, $600 valve covers, etc, etc. that starts running the price up. You can also use them on a Ford Racing block for big cubic inches and big horsepower. I personally wouldn't stop at the 4.300" stroke, but would go with the 4.500" stroke.
Here's a 529ci Boss 9 that I did recently...going in a '70 Mustang, so the cubic inch displacement was a little more important.
B2 Motorsports 529ci Engine Build - Muscle Builder - Modified Mustangs & Fords Magazine
740hp @ 6000 rpm with an extremely mild camshaft...
B2 Motorsports LLC