Most GM engines were assembled from the factory with what's called "selective fit" bearings. These are bearings that range anywhere from .0001" - .0009" (1/10 of a thousandth of an inch to 9/10's of a thousandth of an inch). These bearings cannot be purchased anywhere. Most engines i've taken apart usually were fitted with .0005" - .0008" selective fit bearings. I HIGHLY suggest that you have the crank machined to the next undersize and buy the corresponding bearings so that you don't have any loose bearing clearances.
this is the setup im using for the DZ302 clone im planning on building at some point (although ill probably use milodon street flo valves). Of course you also have the option to spend some more money on porting polishing and hogging the heads out for 2.02/1.60 valves. Vortecs actually out perform most aftermarket heads at low lift numbers, critical for torque production at low engine speeds where, lets face it, youre going to be most of the time. 
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