Agree with smosher. I did this job a few weeks ago. You need to replace the exhaust flange (check the Moyer online catalog) and to do this you will need to remove the manifold. Remove the 3 nuts holding the manifold to the block. You'll need to remove the carburetor first. I cut the exhaust pipe shortly after it left the manifold to make it easier for me to remove the manifold from the block.
Once the manifold is off, you can try to remove the bolts holding the exhaust flange in place (lots of PB Blaster, time, heat), but they probably won't come out. If you have a cutting wheel you can cut off the heads then drill the bolts out (which is what I did) but you need to be careful with the drill so as not to ruin the threads in the manifold, or take it to a machine shop if you're not comfortable doing this. Shouldn't cost much.
Make the system out of black iron. Brass or bronze is just too expensive. The exhaust flange will cost almost as much as everything else.
The other part you'll need from Moyer is the water inlet fitting. I used a plain plumbing Tee, but the Moyer fitting is better as it directs the water flow out toward the muffler.
Well we got the manifold off, but those bots was not coming off, We tried blow torch, pb blast, everything. I'm not up on anything to do with engines, so decided to ask around on how to fix it... Prob is I only have like 3-4 days before it must be working....
I decided to give up all faith and pay my boatyard to clean up old manifold, make new exhaust system, they say they are using high grade stanless steel. and new flange and stuff too.
Just another $500 I don't have, but I just don't have the time of knowledge to mess with it right now.
Thanks for all the help tho guys, you made it so I didn't have to pay anyone to remove anything and get it to a shop..