You would have to do quite a bit to a 318 to get 350 hp.
even with new heads,cam,intake and higher compression pistons the small bore would still probably struggle to match and early stock 340 at 275 hp.
If you build a 318 and 340 to the same specs. There gonna make about the same hp. The bore size isn't gonna restrict the air flow overly compared to a 340 especially under 450hp. A 4" bore is considered the standard small bock bore a rebuilt 318 is only 0.030" to 0.060" under.
The only difference is the 318 powerband will be 200-300 rpm higher and make 20-30 lbs-ft less which can be made up by going one gear set lower.
Saying that if you only looking for 320-350 hp. The 300hp create engine is a stock long block 5.9 with 4bbl and headers and said to make 320hp and any cam and hit 350 hp at least.
