The boat is an 84, so I assume the engine is the same year. And I'm just curious about engines, a 4.3 had pretty much the same hp as a 305 and even some 350s. And they all had the same bell housing bolt pattern. From what I've read, and spoke with the original owners of this boat, it was an absolute pig on gas. So I'll do anything to make it a little more economical.
its a boat, they are all pigs on gas. dropping down in torque will make it more so of a pig and not less and the boat may not make it out of the hole. my recommendation is build a 383 stroker and change props
in 1985, the 4.3 had 170hp, the 5.0 had 205hp, and the 350 had anywhere between 240 and 270. only later vortec heads made the 4.3 up to 205, the 5.0 up to 260 and the 350 over 300.
if you stick with an 85 or older motor you can use your existing flywheel. if you get into an 86 or newer, you will need a flywheel for a 1-piece seal style crank.
if you are going to go thru re-ringing and bering a 305, why not do that to your existing 350? remember, you have to pull the motor apart, install brass core plugs and a stainless steel head gaskets if you are using a truck motor.
I'm guessing a 305 wouldn't do the trick?