All good points made so far. I have owned 6 cyl, 318, 340 and 440 mopars.
Keep in mind your drivetrain is 318 tough, not 440 durable. Your DRIVETRAIN will need beefing up!! You'd need a locking rear diff, etc to TRY TO GET TRACTION!!!!! I used to have 68 GTX with 440. I could do donuts all day long on DRY pavement. I could break the tires loose upshifting to 3rd gear at 60 mph. Went sideways once almost off the road hitting 2nd gear.
What am I saying?? Traction will SUCK with a nose heavy 440. Challengers are already nose heavy. The heavier engine will required changing the front torsion bars, and cornering handling will SUCK unless you add stiffer sway bars. So its not just the engine swap, its all the weight change details that will UP the cost too.
If you'd like more torque, hop up your 318, or build a 360 which is basically a stroked 318, or a 360 stroker if you want to spend more $$$. I'm starting my 74 challenger project from scratch, took the 318 out, and putting in a 360 (no stroker).
My 340 duster did not have locking rear diff, would cook one tire all the time. Ended up adding super stock rear springs and battery in the trunk. Hook up was almost as good as locking rear diff, and cornering wasn't hurt at all, probably improved.
Jay Leno said his hemi mopar had so much torque it was no fun to drive. Not a true road car, he said he didn't drive it much. Hope this helps.
