Do you already have an engine ?
Is Running or doing a rebuild ?
Does it need heads ?
a running magnum with 4bbl headers and small cam would easily get ya there
Or here's a LA stock long block with 4bbl xe262h headers making 282 hp @ 5000 rpm add a little compression 9:1 ish maybe slight pocket porting hit the 300 hp up cam to something like xe268h for 325 ish hp. The speedmasters would be good and probably add 50 ish hp to these numbers. Don't forget stall and gears a 318 gonna need more gear than a 340/360 minimum I'd go 3.73/3.91 with 26"/27" tire size.
318 Long Block Bolt Ons - Tech Articles - Mopar Muscle Magazine