340, 360...

or WHY NOT ? rebuild the existant 318 with some "pepper"... just a cam, set of KB pistons 4 bbls intake and 4 bbls spreadbore sequential throttle carb should make the job, keeping mileage. Something like a good TQ

I don't think will spend more than $600-800 in extra performance parts from a regular rebuild job. These are intake, carb and cam

You could use a resto 340 cam or something pretty much similar from Crower, Lunati, Comp, Engle for the 318 and will wake up the engine without even notice the iddle change, or get worry about convertor or axle gears.

KB hiper pistons doesn't cost significantly more than a regular replacement piston for the overbore job ( maybe just 20-30% more ) so I'm not including these on the extra cost for that reason.

The TQ carb advice comes from the small primaries, and being sequential, will improove gas mileage from any other 4 bbls performance carb. 6139 number was used locally in Venezuela on 318 from assembly plant with success. We got 318 4 bbls engines on 70/72 318 performance cars locally, assemblied by Chrysler of Venezuela.

Basic Eddy performer intake will do the job for the 2 to 4 bbls conversion, and can get it virtually from anyhwhere around.

you could get the engine done in just 2 weeks, depending on your local machine shop