I'm running original 431 Lincolns in mine, they run great but the mufflers began having problems (called leaks). On a 38, at least on mine, the exhaust flow is out of the motor and almost flat, right out the tailpipe, so to speak. Your risers are therefore the only thing keeping backwash from getting in your motor, since they form a dam to keep the flood back when you turn off the ignition. I always wondered if I cut the motors dead while underway, and the boat lurched forward, would the internal water overcome the riser and flood out the motor. Fortunately over the years this has never happened. You have to be careful so your flow is down and away from the risers if possible, so water will pocket down and not sit against the inside of the riser, just waiting to drip drip drip into the motor. A slight upturn to blast through the muffler would appear to be okay. It's been so long since I did mine, I can't recall just how it was done, but it has been working. I used the flow-through resin fiberglass jobs, and never had any problem since. They looked like a giant cherry bomb muffler. Be sure you let the sales person know what you're going to use it for when you buy, I'm sure they sell a lot of them for the 454 size motors.
Dave
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