68 Dodge Dart engine swap??

I recently came across a new project car. A 1968 Dodge Dart 270. It currently has a slant 6 with a 904 transmission.

My question to you guys with a wealth of information is -- What engine can I swap up to? The slant 6 is in rough shape as you can see by the pictures.
I was thinking I could drop a 318 in it relatively easily without having to change anything...i.e. motor mounts/thrust converter/transmission. But after reading some of the posts on this site, a 340 would be a nice score if I don't have to change anything.
Does anyone know if I'd have to change anything if I switched up to a 340?
I managed to get the slant 6 running, but there's just to much to do to keep it going. I figured I might as well put my resources into a more powerful powerplant.
Thanks in advance!!
Kori
[email protected]

dart.jpg

Engine1.jpg

engine2.jpg