When I rebuilt the top end of my old 4.3 after the head gaskets blew back in '16 I knew I was also due for new exhaust, and had been changing them about every 5-7 years. The old OMC one piece manifolds are no longer available (and have not been for years) so I had been researching converting it to the Volvo center riser style which is the same as what you have. I wound up using Barr Marine aftermarket manifolds and elbows, for the hoses I used Volvo OE since they are made to fit the 90* adaptor piece that adapts from the 4" elbow hose to the 3.5" Y pipe hose. It all fit perfect. When running it on the hose, with good water pressure I don't get more than 100 anywhere, except the rear part of the starboard side adaptor piece between the elbow 4" hose and the 3.5" hose, the rear of this part might hit 110 or so. More typically the tops of the elbows and manifolds are 75-85* on the water hose. I'm not sure if there is a difference in the water flow from my Cobra system to the Volvo but as it stands the only difference is the drive mounted impeller vs the engine mounted one on the Volvo. The exhaust and thermostat housing are the same. I think you may still have pieces of impeller somewhere, or the hotter riser is starting to clog. Yeah it can be intimidating to take exhaust apart but us salt guys do it all the time. Your cooler one is right in spec from what I've seen with both the OMC batwings and the Volvo style center riser exhaust. The other one seems a bit hot to me. However, see what you get on the water because not all muffs give good water volume. When running my boat with the old exhaust, at idle they were never over 100*, often as cool as 85*. After coming off plane, the hottest they would get was about 135 and then they'd cool down to 100 pretty fast.
After the rebuild...a pair of reman heads, all new gaskets, and center riser exhaust.
https://www.dropbox.com/s/55j6h4dd1u8fv ... 3.MOV?dl=0Here's a pic of it running on the water hose. My next project is converting from the old Quadrajet to a Holley 4160 as used on the later OMC's from 1990 on and the carbed Volvo versions of the same engine. Don't really want to fiddle with a 30 year old carb anymore.