I've had a couple 340's in the past and that's what my Dart has in it now. But, when I build an engine for it next year (hopefully) I'm going to use a 360 shortblock that I bought for $100.
My car was originally a slant 6, so I'm not ruining anything. I plan on selling the 68 340 block and X heads to someone who is doing a restoration, or "just has to have a 340". I'll take that money and use it towards stroking the 360.
No one will be able to tell if I have a 318 or 408 by looking at the front of the engine, so I may just tell everyone it has a stock 273 in it!! 340s are great engines and lots of fun, but they are also high winding engines. Hard to rev to 7k on the streets around here. I'd prefer to make massive amounts of torque down low, so I'm going with the 408.