There is really no difference in the 383 and 388 you can expect a minute amount more power with all other things equal.
You have some pretty high goals set for that 68 Chevelle, it is not a light car. If you want to find out more info on Chevelles I highly suggest you check out www.chevelles.com
Very good group of guys over there. You should also be able to find a few with a similar setup to get an idea of what you can expect to run.
You didn't say what type of transmission or stall speed so that makes it hard to predict.
If you can hook I think mid 12's should be possible. I don't know about mid 11's. Do you run slicks? If so maybe low 12's on motor.