Slant 6 to 340 Engine Swap in 69' Dart

Throttle cable may or may not work. Engine harness will be different. With V8's there is a 69 down cast Iron water pump and timing tab the and 70-up aluminum pump and cover with tab built in. The outlet is opposite of each other so you need to get the correct radiator for which ever pump you use.

Manifolds or headers? Which intake, as I recall some don't play well with AC? PS pump may not work, get a Saginaw anyway and while your at it get the PS box rebuilt by Firm Feel for Steer-n-Gear in fact might verify the slant PS box is the same internally than the V8.