In the search for a new Fastech I have come across a beautiful 382 with 700 sci. The motors were just rebuilt (due to leaky headers, in freshwater too) and I want to minimize my chances of ruining the freshly rebuilt motors. The owner did not replace the headers after the leak, he just re-welded them. The boat is going to be used primarily in salt water so Instead of swapping on some CMI's for 20k, I was hoping to get some peace of mind with stainless marine's manifolds. What is the general consensus on 700's with these risers and manifolds? Do I need to reflash the ecu's due to more back pressure? While I am on the topic, how reliable are the 700's in salt? What should I be careful of ?

IMO, the Stainless Marine's will be your best replacement option for salt water use. Biggest issue is that the Merc heads will need to be drilled and tapped to accommodate the SM's.
No reason the PCM would need reflashed. The SM's should flow just as well as the CMI's.

Not Sure what Generation Stainless Marines you plan on putting on but I would almost guarantee the Gen 3's would out flow the stock CMI'S

Go with Stainless Marine, you won't lose any performance over your CMI's. Stainless is going to have a 20% increase in price soon, so don't wait too long!
https://www.diamondperformanceparts....ducts/sku17426

How about the hardin 525/600/700 exhaust that doesnt require any drilling of the heads??

https://www.hardin-marine.com/p-8161...-12-pipes.aspx

Yes I was worried about the stainless marine manifolds fitting. I haven’t heard much feedback on Hardins product for the 700’s. What are your thoughts on Hardin vs Stainless Marine

When Stainless Marines Rott out they leak into the bilge, when regular Headers Rott out they can leak back into the Enginess.... as far as performance i couldn't tell you.

No loss of performance with Stainless Marine. Pat installed them on several of the Active Thunders that he factory maintained for customers; after the OEM CMI headers started to fail with use cracks.

I had stainless marine on a set of 700 sci’s in a 39 top gun. The boat ran just as fast as any other stock boat with cmi’s so definitely no power loss. Two new holes need to be drilled and tapped but I think it’s worth it. As mentioned if they were ever to leak, they leak externally not into the engine. Another benefit is the riser/tailpipe is all one piece and is so much easier to remove than a header, one clamp versus 8 header bolts.

I had them on my Top Gun I believe I had 38" centers and they were kinda tight, I'd try to find a set to test fit before I bought a pair. I also Powder Coated mine silver to match the rest of my engines a bit better....