I have a Excalibur 40 with twin 425HP 454's. It had the old 3" rear exit log manifolds that I believe were killing my HP. I aquired two sets of the old merc cast iron perf manifolds from a friend. I have heard they are prone to leaks, but they were super cheap(on a budget). I have a few questions.

The tail pipe system I have for them is off a old staggered scarab. The pipes are 4 1/2. My plan is to cut them down, to slip a 4 1/2 rubber hose to them. I would be running about 16 inches of rubber to the tips. As you can see in the pic, the manifolds were tapped and I can dump some water overboard to help prevent reversion, while still having some to keep the hoses from burning up. I was gonna cut the tails where I circled in the pic. Will this be ok? Would I need restrictors?

Also, I checked all manifold surfaces with a straightedge and they seem good. Will it be a good idea to use some copper RTV along with new gaskets to help prevent leaks?

Also, what kind of HP increase do you think Id get after adding these? The openings where the riser meets the manifolds is huge compared to the old logs, more than twice the size, and it also has indiviual runners inside. I have heard from people to just go buy new manifolds, but that is not a option money wise right now. Thanks for any help!!!

Looks ok from what I can see in the pics, make sure that when U have those manifolds on that the flow is downward and out of the rear of the boat, I had to put some 2inch extensions under my risers to prevent any reversion, from the looks of those manifolds, unless they sit higher than the picture shows, you may have to do the same.

Not sure about water restriction after cutting the pipes. I use 3M black weatherstrip adhesive on every gasket surface with no problems yet. Just make sure not to plug any water passages in the riser gaskets.