You need a bore gauge so you can check the bore for taper and out of round conditions.To measure taper you'll measure at about 1/2" from the top of the cylinder,then about the where the rings would stop in the bottom of the cylinder bore.You will measure parallel to the crank and perpendicular to the crank.Subtracting the top measurement from the bottom measurement will give you the taper(taper is uaually observed in the perpendicular measurement)Subtracting the smaller measurement naturally from the larger at the bottom of the bore gives you the out of round.Personally if the out of round is .005 or more I would bore it,I believe the max taper from top to bottom is .0020 but I have found in the long run with chevy engines that if I have more than .0015 taper I would bore it,they just plain perform better and last longer and not use any oil.
To be honest I usually take the position that if I'm going through this much work,if the block isn't almost perfect(inside of .005's taper and .002 out of round)I bore it but thats me not everyone will be as critical.Many will take a chance shaving a corner here and there but I simply do not like doing the same job twice,I'm just plain lazy I guess.
Use a micrometer to measure the piston and subtract it from the cylinder bore to get piston clearance.
If you don't have access to the bore gauge you can find a telescoping micrometer and outside micrometers at Harbor Freight for about the same price as having a machine shop do it for you.If money is a big factor look into a complete engine kit they usually will save you a little and still get it right in the end.
Take a look at this video when you get to the heads,I was trying to give you a number videos so go to youtube.com and search for How to port and polish chevy heads for more.
port and polish can gain HP and MPG.
