i have a 71' charger that i bought with a worn out 318. i went to an engine shop and the guy flat out told me with a heavy car nothing beats cubic inches. so i went with a 360...bolts right up, might need a new driver side engine mount. so he built me a 360 with 10-1 comp, 490 lift cam, ported and polished heads, air gap manifold and a holley 750. dyno sheet says it should make about 420 hp....im guessing its making about 250-270 hp at the wheels once i put in my new tranny and gears. you could put on a set of edelbrock heads, headers, air gap intake, 650/750 holley. but for that price you could just have most engine shops build you a 360 with stock heads ported. another option is rebuilding the 318 and stroking it to a 402 i think, but that will run you some bucks. so pretty much what it comes down to is for the money a mild 360 is your best bet. a complete 360 shouldnt run you more then 3500 bucks.
and after saying all that if your short on money...like i am now haha...just put in a set of 3.91 gears, youll rev high but you should be able to light up the tires and make sure you have sure grip or get it....you dont want to be smokin one tire in a muscle car.....sadly thats what i do now, and man does that one tire stay smokin for a long time haha