With no power adders the block is more than up to whatever you could try to make NA. I do think taking the bores to 4.125 is just too thin as you're going to want to really spin some RPM's to make great power in an NA motor.
I'm thinking 18 degree heads, sheet metal intake, 4 "stroke and a huge lift custom cam. 700HP from a SB of any kind is going to take some doing NA.
I would not worry about the block at all.
agreed...making 700+hp N/A sounds expensive...lol making 530N/A was bank breaking