depending on what heads you have already you may be able to keep those. I dont know if youre wanting to spend $1,000 or $6,000 but for 350hp a lot of different factory heads will work, including camel humps.
you'll need a good dual plane intake (a $70 swap meet item), good 1 5/8" long tube headers ($200-$400), a good 650-750cfm carb (rebuild a used one), a good flat top piston rebuild kit ($500), and a good cam with springs (often included in a good rebuild kit). Machine work costs can vary but you could have over 350hp for less than a new set of nice aftermarket heads.
350hp is NOT a hard target to hit.
BTW, I see you have headers... All headers are NOT created equal, some are great and some belong in the scrap pile.