What a great idea. Don't let the gasser guys talk it down!
If the adapter plate goes between the Nissan trans and the stock Volvo M46/M47 bellhousing (like the T5 adapters do), so that you bolt the stock Volvo bell onto the Nissan trans, then this should be possible the way you described. You will need to find a diesel M46 bellhousing, preferably out of a 240 to make the clocking angle correct, to put this setup together.
However if the adapter plate you have in mind is designed to adapt between the Volvo engine and the Nissan bellhousing (rather than the Volvo bellhousing and the Nissan transmission), it will not be bolt-in and more work would be needed, since the Volvo diesel and Volvo gas engine bellhousing patterns are different.
To my knowledge the diesels only ever came with dished flywheels. However the clutches and pressure plates are otherwise similar to the gas Volvo setups so anything you can do to beef up one of those would work for a diesel clutch as well.
Welcome to the board, keep us posted on how it goes and be sure to share pictures.
