I know nothing of engines, so
what does it take to build a 350 /260-280 hp - it has dart heads now up,to 300-320+ hp.
Is it worth it.
Cost?

Not to sound like a smart@$$ but if you know nothing about motors you should probably find a reputable shop in your area and ask them ......"how much"...
You can back up if you don't know what you are doing with an engine

had a 2000 250 hp 350 in a 21 foot boat.swapped cam,intake and carb woke the thing up big time.i'm guessing 320-350 hp.was together for @5 years not a single problem.depends what your lookin for and how much you want to spend.

mike i'll pm you back tonight got some ides for you to make that a runner

I'm a little confused by your question so I'll try to clarify things.

IF you want to build a 350 cid motor that makes 260-280 hp that should be easy and not terribly expensive.

IF you already have the motor and it has Dart heads then that's great, but it would help to know which Dart heads (intake/exhaust valve size and material, intake runner size in cc's). 300 hp is not difficult. 350 hp is easily attainable. 400+hp is doable but things start to get expensive. So in the end it all depends on how much you want to spend.

If you can afford it stroke it. You will benefit from the extra cubes and torque.

Many factory Merc small blocks in the 350 cid class made 250-265 hp at the prop on fairly mild cam set ups.