You'd be off to a better start, not sure if your bellhousing (or flywheel to spline drive adapter inside bellhousing) or engine mounts would fit. We'd both need a marine exhaust suitable for the LS3, engine management electronics, probably a closed cooling system.
I haven't looked into closed cooling systems much, not sure how much they cost but I know you can either run just the engine closed system or the engine and lower exhaust manifold, cheaper if just the engine. Would allow dropping in any old Ford small block without needing to swap bits over such as marine inlet manifold, water pump, distributor, core plugs, etc and the engine would be a keeper instead of semi-disposable due to being eaten from the inside by salt water (or even fresh water without corrosion inhibitors). Then in time could swap in better performing bits such as aftermarket aluminium heads, intake etc. The heads on these old Fords are the main limitation for performance (followed by the rods), the old cast heads can be gas flowed for a minor increase in performance, early 90's Mustang 'GT40' cast heads a bit better still, but much better aftermarket aluminium heads are available and could be fitted in future for a decent performance boost even without a longer cam. I think the standard old GM engine heads flow better than the standard old Ford heads though, your Vortec spec heads probably a bit better still.