Welcome aboard
the issue with your 229 is that it is a 2-piece crank seal where most 4.3 motors you find are 1-piece crank seal. only the 1985 4.3 would work for your application. the chances of finding a low-mileage 1985 4.3 that would be good enough to use is slim.
you could find a pre 1986 5.0 and use that, however you will also need to change the gear set.
the only other alternative is to either rebuild your existing 229 or to get a custom coupler made. taking the OMC coupler for a ford motor and re-machining is the only viable way of getting a coupler for a 2-piece crank seal for your OMC stringer.
if you use a 3.8 out of a 1984 or older GM Metric chassis vehicle, you still need to pull the motor, strip it down, change the cam, the core plugs to brass and the head gaskets to marine. make sure you use all your existing marine accessories.
out of curiosity, what is wrong with the 3.8 you have in the boat now?
