Conventional wisdom says Merc GM engines are nearly identical to their automotive counter parts, and I believe this is true. My 4.3 LX parts manual gives a long speech about only using merc. parts. Surprise, huh?
So I'd go w/
Scott Danforth and tear yours to the ground. Buy a solid 4.3 Vortec junker w/low miles from a wreck, and reuse as many marine parts that are good. I'd say use the salvage cast iron, and reuse the rest, including crank, pistons, rods, cam, valve train etc. in the salvaged heads, and rebuild the heads with your marine parts Buy new frost plugs, and any other sea water contact parts that you need. May as well do new rings and all seals as long as you're this deep.
Engine pull is intimidating, but not so bad after you've done it. Pay attention to the rear engine mount hardware. The spacer sleeve is easy to forget or mis position. The fiber washer/lock washer must be contact cemented on the gimbal plate, or it will likely fall into the bilge when you try to start the bolts. Be sure to re tape the nuts into pocket provided. Don't move the adjusting nuts on the front mounts, else you'll be wondering about alignment. Although it is a good idea to recheck this with the alignment tool when you're this deep.
That sea water seal in the bell hsg. is ugly. Scrape and sand off enough corrosion to get a flat face on the bell hsg, but don't go past that. I contact cement the new O ring in place so it doesn't move, and cause this.
