Do you have a 340 running and ready to drop in?
Is the 318 in good condition?
318 are given the “Dog” moniker because they were often low compression engine (7.8-1) saddled with a lot of emissions devices and low gear ratios, 2bbl carbs and hwy gears. Early 70’s and ‘60’s 328 had a little more compression and work well with small camshafts.
They are very good in obtaining mileage if not given to much cam and gear. AJ/fromS suggested an OD trans which is difficult to come by. But super helpful for mileage concerns. His suggestion of a 360 and I’ll add it in a 5.9 for more power over a 340 is a wise choice since a good running 5.9 will make 300hp with just an intake and headers. You’ll just need to address the balance issue with a proper flywheel or weight on the flywheel.
FWIW, my 5.9 gets only 17 move but it is in a bigger car, a ‘1979 B body Dodge Magnum w/3.55 gears.
If you want to hop up the 318 AND you’re doing a rebuild on it, the only expensive part that differs from the larger engine that is expensive are pistons. So if you go out and get a 340 block, that cost in getting the 340 block alone will be more expensive that the 318 pistons.