Engine build.....

Ok well im going to be going to the junkyard again here in a few days....(finals week got to get test done)....and was wondering, should i go with a 4g64 or 4g63 block on my build up? The engine is going to be have turbo, either way going with a dohc head, forged internals, and 5-speed swap, and all the other goodies....I'm not sure how much boost im going to run but idea is around 20psi so for a boost application would the 64 or 63 better? They only have longblock 64's out there and im not sure if they have any 6-bolt last time i checked they had one but someone was looking at it so someone might of taken it...