340 in a 65 plymouth valiant

Pretty easy swap. Motor mount modifications (weld on a triangular tab)or use Schumachers, Dropped centerlink is required, Timing cover and water pump combo with radiator are a minor concern since the water hose hookup is opposite, it's just a matter of selecting a correct combination of parts (you can swap 273 timing cover and brackets like 396 Signet did), bellhousing/clutch is another area that can be a minor puzzle. You have to decide if you're going to keep a 273 122 tooth flywheel and 9-1/2 -10" clutch or change to the larger belhousing and larger 130T flywheel clutch combo. Correct linkage can be a puzzle if you swap the the larger but a lot of folks have made the change so make a decision and ask for info on the combo you want to run or you can buy the correct combo from Brewers. As has been mentioned, exhaust will be your biggest issue...but there are a few options.....
Have fun!