I've a 4.3 in a runabout (2006 Caravelle). Perhaps some of the more senior members will post as well. To my knowledge the blocks should be interchangable. I do know that some of the larger v8's that come from companies like Crusader are equipped with 4 bolt main blocks, but I can't believe that this would be an issue with a v6.
When I did my first oil change on my 4.3 I found an AC oil filter. The filter was half black from the paint that Merc used on the engine. Behind the filter, where the black paint couldn't reach was the light blue factory paint from GM. That leads me to the conclusion that the block is purchased from GM and then "marinized".
I can't see why engine mounts, manifolds, etc. should not just swap over. You'd want to use parts USCG approved for marine use of course. I'd take a good long look at the exhaust manifolds. They are likely original and at what? 23 years old they have probably reached the end of their service life.
I'd check on the EFI though as I'm a carb guy and not experienced in that area.