I have been "working" on*marinizing a pickup truck Cummins 6BT so I have some experience on the subject.
While still in the parts gathering stage, and have been for about 3 years, I can tell you that some parts are easy to get ahold of and some are not.
I started with a low miles (approx 100K) engine out of a 1989 "farm use" pickup. It ran really good.
I shopped and found a set of 210 marine injectors cheap and also a "jacket water" aftercooler with fuel lines for dirt cheap. I traded some of the pickup truck parts for a marine exhaust manifold.
Last falll I bought a Cummins 6BT that was in a boat fire. I am sure the block is good but I really bought it for the marine parts. Turbocharger, another exhaust manifold, flywheel, bellhousing, raw water pump, mounts,**etc.* Some of the parts need a rebuild but now I have the cores.
I need a few more parts before I start piecing it together but I still need a heat xchanger, tranny adapter plate, etc.
However I have seen the long blocks that FF mentioned so I need to inquire about those before I do anything else. I need to find out if the LBs come with injection equipment or not. I would guess they do not as there are multiple pumps that Cummins has used depending on application and HP.
Oh yes I also bought a BW 72 series (2.57:1) tranny for cheap to put behind this engine that I will tweak to approx 220 hp using the truck injection pump from the 89 engine (which is a Bosch VE pump. Not normally used for marine use, a Cummins "guru" in England has used this successfully and I have his fuel delivery formula for it which he says will run rings around the standard Cummins 210 with a CAV pump).
So if you are thinking about buyiing a LB to start a marinization project there are some things to think about other than just labor.