For the price of building an iron block LS motor I wouldn't even bother with a SBC, they can be made to work well and are tried and true but starting from scratch especially on a budget the LS has more bang for the buck IMHO.
I helped my Dad build the 302 for his Mustang, and it's pretty similar to a SBC overall. They are more expensive to build then a SBC, to get good parts it's about what an LS motor costs. If you are starting from scratch then get a 351 windsor block as a better foundation, its just a better foundation.
So its not just that I'm partial to LS motors, but they are so much better stock that to get the same power out of the SBC/SBF compared to a bolt-on cammed LS, you'll have more money in it. You'd have to buy some nice heads for the old motors just to keep up with stock LS1 heads. You could buy good used parts for an LS motor, that cleaned up and installed correctly, would make good power, and most people think stock LS1 heads, stock cranks, rods, even pistons should be replaced for power. It just depends a lot on how much power you want to make.