Re: sea sprite 4.3 v6 engine swap
it is possible with some things to look for.
you have to strip the pickup motor down to block and heads, install brass core plugs, and I suggest a marine cam.
you will then need a vortec intake and to bolt on all your marine stuff from the '84
you will need to find and wire an electric fuel pump (vortec motors in cars and trucks are not machined for fuel pumps). this will have to be wired with an oil pressure switch so the fuel pump stops when there is no oil pressure
you will need to have the right flywheel. GM switched from a 2-piece rear main seal to a 1-piece in 1986
you did not state which drive you have. you may have a problem locating a coupler and flywheel, especially if you have an OMC drive