if you understand righty-tighty, you can do it. simply unbolting parts, cleaning, bolting other parts back on, stabbing the dizzy and adjusting a carb.
most places will gladly take your money to work on things. however if your paying someone to turn wrenches, then make sure you have deep pocket books for this already expensive hobby.
you could swap heads and intake yourself, find the parts, order gaskets on-line, etc and have the job done for under $500 if you shop around (reconditioned heads, used intake, reconditioned carb), or about $1200 for new parts (new heads, new intake and new carb)
paying a marina to do it means you pay list price plus their book labor. that $480 set of new cylinder heads ordered on-line and delivered to your house lists $1530 each at a dealer - that is 3060 for the pair. and 4 hours to install each that $230 new intake from Chevrolet or Summit lists about $530 at a dealer and 3 hours to install that $15 set of valve cover gaskets list $30 each at a dealer..... you get the idea. paying a dealer to update a 4.3 to vortec heads with a 4-barrel will cost you a ton of money
at that point if you lack the skills, buy a boat with the 4.3 4-barrel or 4.3 MPI to begin with.