Looking to get more out of my 350 mags in my Formula Fastech. Would it be worth doing cams, heads, valves, rockers etc? Or would it just be a waste of money? Any input is appreciated!

Everybody has a different financial tolerance for "is it worth it" regarding engine mods. I would break the thought process more. Can you get more out of a 350 Mag? Yes, with appropriate cam, heads and associated valvetrain as you mentioned. You could also add in the option to stroke the motorv for more CID which is more power. Its tough to put a price tag on that but upping the motor from say 250-300 hp to 375-425 hp using the above mentioned mods and including appropriate machining and assembling will cost between $4000-$8000 per motor. Then you could research how much it would cost to swap to big blocks? Problem with switching to big blocks is you have to start over with a lot of new parts that don't swap over from the small block, hence the cost jump is often significant.

In the end you have to ask how much are you willing to spend?

Is it a waste of money? Crap, most could argue any boating related item is wasted money but we do it for enjoyment. For me painting a hull with latex house paint would be a waste of money. Modifying an engine to see what happens to performance is not.

To get any measurable and seat of the pants performance improvement will require a large chunk of money.

383 would be my go-to if I was gonna do an entire build. I built one before for less than 5k and made 465hp and 450 ft lbs that's gonna make it's way into my 69 Chevy pickup. I was curious if anyone has just done top end work on a stock 350 mag short Block and what their results were.

before considering big blocks, which apparently you are not, one must measure the center-center distance between the engines-drives. 33 inches is the minimum for big blocks. most twin small block setups are skinnier than that...

A bone stock LS3 makes 430hp.
You will need exhaust manifolds because of the different design ports.

Then sell your 350's to any cruiser looking to upgrade to efi or replace a bad motor.

Don't forget the weight savings of the LS3 and the lighter manifolds alone will help on the ass heavy 292.

Do it right and you could almost be a wash for the extra power and cool factor of one of the only LS3 292's

This is my plan at some point in the future.

Supercharging will be the best bang for your buck.

that would be really the only or one of the options I'd consider. I built one in high school for my z28 and was a power house with very close hp and tq as your. I used the comp 292 magnum cam advanced 4 deg. Actually pretty reasonable nowadays to build also.

What Outdrive do you have?😳

Has anyone used the LS3 or LSA in an offshore sport boat in the 30’ range?
it seems to be a great upgrade from twin mag 350s without the weight of a big block