If you are looking for a like new set of manifolds/risers I have a buddy that just replaced his with IMCO. His Mercs are like new in/out all fresh water.
I helped him remove so I have personally seen them. Let me know.

While you are there I would also replace the block drain petcocks. I prefer this type as it is almost impossible to get to them but with a 9/16" wrench you can reach them be assured of being able to tighten and loosen. You will need 2 ea. of them.

https://www.ebay.com/itm/Genuine-Mer...-/153107583902

I just ordered some for my engine. They are much nicer than a regular steel plug.

Ok I'm not a mechanic by any stretch of the imagination - I have replaced a manifold on our 7.4 mpi which went very well but that doesn't make me an expert. Just a question - if you are replacing the manifold - the ones that are coming off will be scrap- right? why not just take a saw and cut off as much as you can of the old manifold so that you can get a socket at the bolts that are hard to reach. in other words make room for everything - after all you are throwing the old ones out. Then when it gets time to put the new ones on - start with the bolts that you can get your hand to first and tighten them up. Again I'm not a mechanic and please don't shoot the messenger.

3pointstar

My 2000 260ss 454 mpi manifolds were mounted with studs & nuts so I used a dremel to cut off the nuts, slide manifold off, back out studs with pliers/vise grips (all came out easy) and replaced all.

How goes the battle?
How bad are your manifolds/risers?
If they have much rot/rust you really need to look at the iron intake manifold. Mine was almost ready to break through just below the thermostat housing, I caught it just in time.
Keep us posted and good luck.

Where are you located?