Your pretty much discarding the entire bottom end of the motor and running new crank , rods ( your old rods may or may not get reused ) , and pistons . Your block will need to be bored , honed , and ground a little to keep the goodies from hitting it in a couple spots . Lots of attention to detail during assembly .
Then of course its all the labor to remove and reinstall , and its going to need to have the supplemental parts and tuning for it to run right with in the car with the extra cubes and compression .
Most guys without patience , time , or skill just buy one , or have the original one done at a shop .
BTW , I am all for keeping the original numbered block in the car :biggrin: