appearance of a 340 vs. 360

As said before, none of the production block 340s had 4-bolt mains. But both your '69 and '70 340s will have forged cranks vs. the cast crank in a 360.
Also both of your 340s will have 2.02" dia. intake valves, but you can also build 360 heads with 2.02 (or larger) valves.
The 340 T/A blocks had a higher nickel content and filled oil pan rails for a bit more bottom-end strength.
I love the 340 engine but I'm building a 360 for my '69 Dart because it came from the prior owner with a crate 360 in it.