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Sadly, Crusader stopped supporting the v-6 products a while ago...
As far as alternatives go, about the only choices I know of are Mercruiser (original or dry-joint) or their aftermarket equivalents. I'd be inclined to buy Barr before Sierra and avoid GLM altogether)...
I'd suspect if you have to change the manifolds and the elbows, you are likely going to have to get new exhaust hose, too...and getting new clamps would make sense.
You can always check out what our host has to offer...I'd search the mercruiser parts listing for their 4.3 engines from the mid 80's...
OK...then I'd say you have the old inline 6 GM block...where you get 230/250/292 CID versions.
you could try this:
https://www.marineengine.com/newparts/part_details.php?pnum=BARMC-1-58759
If the formatting is off, just copy and paste into a new browser window...
you'd have to get 'finishing parts' and make the plumbing work - not hard but detailed...
https://www.marineengine.com/newparts/part_details.php?pnum=BARMC-1-99746
the v6 gm 229CID engine is also known as the 3.8L, not 3.9l
The marine engine 'OEMs' typically used GM standard production engines that they added their bolt on pieces to to finish the marinization process. If you identify the correct engine family, there's a lot of interchange...you just have to be careful in that changing one part can cause one to replace several others.
the cast iron elbows around here usually don't see more than five seasons...some report issues after 3 seasons...a lot depends on the crud in the water, the quality of the casting, and the use the engines see...i would say the elbows are likely to be clogged up at 7 seasons...and the likelihood of cleaning them up with damage is very low...
and a clogged elbow will flow very little water and, if run like that, will normally burn thru the hose the elbow dumps into.
the exhaust manifolds will normally go a couple decades, even with raw water cooling.