Can I drop a 340 into my slant six and still maintain the rest of the drivetrain?

No, small block and /6 bellhousings have different bolt patterns.

Copy pierre's big negatory on that one.
Also, add into the parts list a bigger radiator, k/member and mounts or conversion mounts, t-bars and bigger better brakes cuz you gotta have the whoa, to stop all that go. Also, you'll need to modify/build kick down linkage (or buy one of those slick cables).
It's all cake baby. You'll need a proverbial butt-load of beer, but the swap is easy.

-JYH
69 Cuda: +2 Cyls
64 Dart: exploded view
SoCal Baby, SoCal

What year vehicle are you working with?
Other little items needed for the V8 swap include the special exhaust manifolds (model / year specific) and the rear-end. the 7 1/4 rear end found behind most SL6 engines don't hold-up well to increased power and torque.
The best advice for doing this kind of swap is to find a V8 car of the same year and model series to use as a "donor car" for all the parts you will need.
DD