SVO / Ford Racing blocks that can be bored to 4.125 are siamese bore. There is no water jacket between the bores, and more room to achieve a larger diameter cylinder. There is some net horsepower gain with the larger bore, claim most engine guys. To my knowledge, the siamese blocks are all 4 bolt mains. The 351s are available in both 9.2 and 9.5 deck height. The 302 are available in 8.2 and 8.7, to the best of my recollection. Most of these blocks require final machining, bores, mains, lifter bores, etc. They're great, will hold up to a ton of power, but are not cheap.
RD