I run a 3-71 Detroit in a very old work boat. It uses a Paragon HJ6 hydraulic transmission with 2:1 gear reduction.
There is nothing special about mounting the transmission on a 71 except it is heavy.
The transmission simply bolts to the flywheel cover, the transmission is driven by a splined input shaft from torque disk bolted to the engine flywheel. It could also have some adapter rings to mate to the transmission bolt pattern.
If the engine has been submerged in sea water
it is probably a rusty mess.
Is the engine a modern turbocharged innercooled engine or is an antique like ours. I like the old engine but it does run smokey,
How do you know the cam is seized, on a detroit the gear train to drive the cam and balance shaft are always on the front of the engine under the gear cover. Take the inspection
covers off the opposite side side from the blower and check to see if the pistons are badly seized in the bores, pistons can be viewed via the intake slots in the liners.
I recommend you get a set of service manuals
for everything you have.
Ebay is a good place to start looking.
Don't know where you are but technical support is getting harder to find every year, but it is a simple machine and I have always maintained ours and parts
support is still good.
Boatdiesel.com has a detroit section with several knowledgeable supporters.
Good luck with the old detroit, cast iron monster engine.