Editing Decking a block

Setting the squish is a must-do if you expect optimum performance on pump gas. It is generally accepted that 0.035" to 0.045" is pretty much ideal. You can do it with a 0.025" piston deck height and 0.015" gasket or you can do it with a 0.012" piston deck height and a 0.028" gasket or you can do it with a zero deck height and a 0.039"/0.040" gasket. 
Don't cut the decks until you have all of your reciprocating assembly parts on hand to measure. And be careful when you buy pistons for a 350. The preferred ones will have a 1.560" compression height. "Rebuilder specials" will have a 1.540" compression height, requiring additional meat to be whacked from the decks. If you are planning on using cast iron heads, then the thickness of the gasket makes little difference. If you are using aluminum heads, you must use a thicker (0.039"/0.040") composition gasket to prevent brinelling the cylinder head, so you're generally talking about zero-decking the block in this case. Fel-Pro 1003 works well on a 350 aluminum head install.

Something else to consider is the alignment of the manifold and sealing it up. When you cut the block decks off the centerline of the main bearing bore, you have gone a long way toward having the heads square to the motor. Sometimes when the machinist is cutting the block decks at the factory, a chip might be left in the register and the block will be machined uphill/downhill from one end to the other. You can see how this would not allow a symetrical alignment of the cylinder heads and thus improper alignment of the intake manifold. So getting the decks square is a good thing. The machinist will know how much to cut off the manifold to get it to sit properly after decking the block. Make sure he cuts the china walls of the block too, to prevent stand-off of the manifold in the event you or the next builder uses stock end seals. I normally throw them away and lay a nice fat bead of silicone on the china walls to seal the ends of the manifold, so I don't worry about the height of the walls.

Please read this wiki article about manifold alignment also. It may help you seal up your motor....
http://www.crankshaftcoalition.com/...nal_vacuum_leak

If you're planning on using cast iron heads, you may want to cut back a little on your static compression ratio. You can make a real strong street motor with 9.5-9.7 SCR and the slightly lower SCR will allow you to use full ignition timing without breaching the detonation threshold of the pump fuel you're using.

I haven't offered up ryanscarpage to anyone recently, so it's time for a re-visit. This is a list of 108 dyno tests that were done on different combinations of small block Chevys. All of them are meant to run on pump gas. At the bottom of each page, click on the next group of tests until you have been through all 108. Some of them will not apply to your build, such as the ones using a 400 block, but they are all interesting, nevertheless....
http://www.ryanscarpage.50megs.com/combos1.html

One more thing. Measure all volumes so that you can figure the static compression ratio for yourself when you have finalized all your parts buying. You will need a lot of information including the exact SCR in order to intelligently choose a camshaft. All of the cam grinders have a free service where you call them up and tell them all about your build and they suggest the perfect cam for your motor. Use this service. 
If you don't know how to figure static compression ratio or which volumes are involved, just ask me. I will be glad to explain it to you.