Need advice: 318 or 340 for a 68 Dart?

We haven't pulled it apart yet to see what's going on with the brakes it was stock drums all the way around when I had it. Still sporting my rally wheels from my 16th birthday so the stock wheel size hasn't changed. I'm assuming it still has the original rear end and all 6 cyl standard components but I can't be sure because someone put a floor shift in it even though it still has the original steering column shifter on it. Based on seeing that, everything could have changed. Maybe they blew whatever bigger motor they put in it and that's why the motor's missing. A lot of unanswered questions yet.