ok there are a few problems i noticed with your list.

1.360 engine all the smog bullsit off.
ok no big deal.

2.magnum heads,non cracked,milled 30 over ported slightly also.thin head gasket.
i wouldn't spend the money here just yet. get the pistons first and use the stock heads. like mentioned before the stock magnums are pretty good flow wise but the seats tend to develop cracks. i would really wait and put aluminum heads on it anyway.

3.headers
yes good cheap headers will be fine for a budget build.

4.holley street dominator sigle plane alumnium intake,4 barrel 650 cfm carb
no single plane intake. this is a bad move for a 360 you definitely want to stick with a dual plane. the 650 is a good carb, but i think a vacuum secondary would be perfect.

5.comp cams 270 magnum cam
this cam maybe a little big with stock pistons and may bleed off too much cylinder pressure.


6.gm V6 valve springs
don't need these if you run the stock heads. i don't know what the lift is on the 270 cam but it probably has a set of springs that are recommended and i'd get those.