Well, since no one else is jumping in and I am doing absolutely nothing right now I'll say what I "think". Just my opinion.
First you can buy new blocks. I am told they are weaker than the old 69-70 something blocks. The new ones, Lightning, do have provisions for roller cams, I think. As for you powergoal, I'm sure you could split that block in half with just twin turbos and 351 cu in. 700 hp isn't that hard to achieve with twin turbos and a few cubes. I do think 700 on a 351 can be a bit risky without some addition strenghth items such as hardblok and maybe a girdle, which is still in debate. Scat is making some great, cheap, and strong stuff for the Windsors. Give them a look over. Cylinder heads, well how much money you got? Seems all the companies are making some great off the shelf stuff, AFR and Edelbrock seem to be what everyone is running. I don't think you would need race gas with 700hp, but it would be good insurance. Maybe a mix of 91-93 and race gas. I'm curious to see what others have to say about this.