you guys mention an extra grand laying around! The DART SHP block is 1600.00 (not including the 100.00+ s/h) and the DART little M is like 2500.00 (again not including s/h). Unless you are planning on running over 600hp or planing on running a 3.875+" stroke then I dont see the point. for the extra money you spent on the block i could tear down the motor, buy another block, have it machined and likely buy a few rods or the crank to replace the one that broke. Im all about saving the extra cash to make sure you do it right, but i dont see where an aftermarket block is needed.
to the OP, a 350 making 500hp NA isn't too hard, but one making 500hp NA on 93 or less octane is. it will want to turn some rpm, so a decent forged rot assy will be needed (IMO) you will want to push the limits with compression, say 11-11.5:1 with alm heads that have around 210-215cc intake runners, spend the extra cash and have them cleaned up, 112LSA hyd cam with around 245° @ .050 and .575" lift, use 1.6 rockers to bump you over the .600" lift mark. use a single plane intake like the vic jr, and a 750 double pumper. matched in a 3500lb car with a 3spd (th400-350), 3600-4000 stall and 4.10's to 4.56's.
It wont be as much fun on the street as it will on the track, but you should be able to get it to run on 93 octane.