I have read lots of stuff and seen test with Gils, Stainless marine etc on 500hp engines but what about on a NA 650-670hp bbc 540? I know the manifold-type GIL/Stainless/Seaward are not as good power-wise as real headers like CMI but they are MUCH cheaper and it’s easy to find dry tails if needed. The engine is going in my Formula 271 fastech and replace my old Merc HP500 (carb). The Hardin Marine Seaward system with dry tails is about 2600dollars and the Keith Eickert PF cast headers are 3600 dollars with tails. Used fresh GILs with dry tails are like half price compared to the new Hardin Seaward. Of couse money is an issue as always.

Gils will start choking power at around 600hp.

Gils start choking much lower than 600. It has more to do with RPM. Shirt runner stuff dies at about 5000. You definitely want longer runners. Also look into the thunder and lightning manifolds from diamond. They are long runner manifold. Much better priced than stainless headers and perform extremely well.

CMI Fusions, they are dry to the end of the header, well all but about 4". Made to be salt friendly with care.

Stainless marine...

They found 50hp in this 540ci bbc going from 1.75" to 2.25" headers. I'll bet if they had tried a set of "stainless marine" or "gils", there would have been a 50-75hp gain going to 2.25 headers. This is why you see where a guy dyno's his 540 and makes 650hp with big 2.25" dyno headers, no accessories. He then throws on a set of GILs or similar, and installs all the accessories and drops them in the boat. Now hes unsure why his boat isn't faster than his buddys exact boat that has stock 575 mercs

https://www.hotrod.com/articles/hrdp...chevy-headers/

You can make all kinds of cool high rpm hp in the 650 hp + range on dyno with headers. Making it in a boat with anything less than best marine manifolds is hard. On my dyno, tested a strong 509, made 695 hp w open headers, around 680 w mufflers and in the 650s w emis with dry to tip tails (going on memory). Peak also happened 300 or so rpms EARLIER with those manifolds. In previous testing, those EMIS outperformed a set of imco poweflows by 10+ hp and ft lbs of tq so imagine stainless marine 1s, gils, imcos etc on anuything much over 500 hp are choking off motor.
So, this really needs to be kept in mind when choosing a camshaft. Very easy to cam a boat too big if exhaust sucks. Of course, we all know this already but heres a example, I'm just going to make some numbers up to illustrate my point but they will be close.
Lets say you pick cam A, it has 242/248 @.050. On dyno and dyno sim your motor torque peaks at 4800, hp peaks at 6000., makes really cool power. Now, you put it in boat with seaward exhaust, peaks at 5600, you've given up usable torque in the 3500 to 5000 range to make a bigger peak number but it doesnt make that number anyways in boat. Cam B is 8 or 10 degrees smaller, it makes 20 more ft lbs of USABLE, average torque all through power band, peaks at 5700, in boat pulls to 5600. Actually pulls prop nice too. Your on right track thinking about manifold options as there is very little that doesn't choke off hp. Im buying a set of thunder lightnings to try on dyno next for myself, similar build,Smitty

My son has a 25 active thunder early 90s non stepped V hull we built a 540 for it it has painted tube headers with dry tails with well north of 750 hp its one of the most fun boats i have ever been in and i have been around since the first go fast craze late 80s early 90s if you are building it for sheer pleasure of hearing a bigblock singing and sounding clean and crisp then go for tube headers its worth the pleasure! we can turn it up to 7000 but most of the time we keep it under 6500. there is a distinct sound you can hear when it fires up no matter where you are in the marina go for it !

call Bill at Diamond performance.

this ^
had Gil’s switched to the Diamond cast header, boat runs and sounds better. 6mph at cruise and 4 top end, tho I’m sure prop tune helped those #s
540 making 646/690 on dyno