my chevelle came with a 215 horse 454 truck motor in it.What would I have to do to bump up the horsepower on it?. I plan on using the heads that are on the motor which are a 781 casting.113cc chambers 2.06 valves.I know these heads are not the ones that came with the motor. I want to use the crank and rods that are in the motor and I have a crane cam .449 gross lift 266 duration.edelbrock 2-0 performer intake and 600 cfm carb. could I achieve 300hp by using these components and just changing the pistons? I want to leave it the stock 4.250 bore if possible.

Your motor is about 8.0 to 1 compression
Crank and rods are fine.
Closed chamber heads are your best bet if you leave the lower end stock.This will bump you up to about 8.9/9.0 compression,and give you more room for cam selection.
Your heads are more like 118-122 cc's
Good heads,but not good compression with stock flat tops,thats why closed chambered heads (101 cc's) wake these motors right up.
A Performer RPM intake,a vacuum secondary 750 Holley and small tube headers will get you over the 300 HP mark easily,even with the stock cam,probably closer to 350.Torque is about 450 or so.Read this:
http://www.hotrod.com/techarticles/engine/113_0110_454/

How much bore wear do you have? THAT is what determines if you need to rebore or not.

I have not got the motor out of the car yet.I am probably going to stick a set of closed chamber heads on it and leave the bottom end stock.I have 2 sets of 101cc chamber heads at my disposal.I will also use a edelbrock 2-0 intake and 600cfm performer carb along with a crane 266h camshaft. hows that sound guys?

The Peformer will work fine,and so will the cam,but actually,something a little larger would be better,like the Crane 272H,especially if you go with the closed chamber heads.
A 750 vacuum secondary Holley would be a better carb choice.I ran a 600 on my 454 when I 1st installed the motor..I used a 50 cc pump and the lightest secondary spring (short yellow)
Worked great,but no comparison to the 750 in terms of performance.