I own a 1980 Plymouth Volare coupe. It has the 225 slant six in it but I'm looking to swap it with either a 318 or 340. I have a 318 lined up for me with the trans for $250. But I'd have to do more **** to it before throwing it in. But I'm thinking a 340 would be better to go with. Just curious to see other opinions on how I should do this since I'm new to this. I basically want it to have more power and ***** hah. Not looking to turn it into some hotrod or anything like that thought, this is pretty much my everyday vehicle.

 

not a simple swap

 

It's actually quite easy, It just gets involved & $$$
Change Radiator, engine, trans, drive shaft Rear end & special motor mounts

 

If the slanty is an automatic, the drive shaft and rear end would still work. The shift linkage is the same on all 904s whether the car was a slanty, a 318 or a 360. The outlets on the radiator can be relocated: a good radiator shop can do it fairly cheap.
318 or 340? Hmmm, If the goal is for some mild street fun, a 318 with some cheap upgrades will get you there. Add a 4 barrel and either a Thermoquad or a Holley 600 and add dual 2 1/4" exhaust pipes. If the car has a 7 1/4" axle, don't do any 5000 rpm neutral drops...That is the weakest axle ever put in a rear wheel drive Mopar. The 8 1/4" axle is a huge improvement and they are not that hard to find.
340? Too expensive for me. You would be competing with the restoration guys that need them for their Darts, Demons and Dusters. The 360 is almost as common as a 318 and it is almost as cheap to build one. My brother in law has a 72 Duster with a 360. 4 barrel with a 340 spec cam, dual exhaust and 2 1/4" pipes. That car will lay down a 50 foot burnout with the 3.91 Sure Grip axle!