A 383 will make a whole bunch more torque at below 6k. Our first marathon boat ran a 383. We built it to stop pulling at or near 5,800 rpm and it ran quite well. We were running a B/C impeller and as far as "running away from other boats like they were tied to a stump", you can probably get that thought out of your head.
I think you will find that a properly set up big block with modest HP will stomp on you like a piece of chewed up gum.
If you really want to get a true sleeper, I would look into an LSX. The bell housing will work, but you will need to get some creative juices flowing for the rest.
An LSX can make almost twice the HP and torque as your 383 with several times the reliability.
Good luck with the build and welcome to this thing they call jet boats.
GT
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