First, I did search this site for info on my particular issue, but did not find the specifics I looked for:

I have a Donzi Blackhawk (BH) that I am powering up. As you might know, the BH 502 Magnum does sit higher in the engine compartment, so my question can't be directly addressed on the basis of a Classic 22.

My conundrum is I expect to dyno appx 650 hp, normally aspirated and am looking for exhaust options. I have looked at many marine headers, but they all measure to push through the hood, which I don't want to modify. Basically, I have 1.5" above the standard Merc 502 exhaust risers - that's it. From the crack center, that's a height of about 21-1/4" available. Most headers are higher. So, I thought I'd look at elbows, but there are so many out there....any suggestions? Thank you

I would call Stainless Marine.

I had stainless marine make me custom risers that had to fit exactly to the existing holes cut in the transom. Taking the measurements they needed was pretty easy and they did a great job at a good price.

In this picture you can see how close they got my risers to the existing holes in the transom.

I'd recommend them.