Vs: Mitsubishi L300
I'd have to say no, the gearbox from 4G32 will not fit to the diesel. The diesels have a different bolt pattern (engine to gearbox).
The easiest way to do the swap is to have a wrecked diesel van next to yours, that way you can take everything you need.
You have to prepare yourself to change/work on at least following things:
gearbox, clutch, flywheel, starter motor (all different on diesel)
propshaft, possibly gearbox support cross member, shifter and linkage.
exhaust pipe, alternator (diesel has one with a vacuum pump for the power brakes), engine electrics (tricky, diesel needs glow plugs and electronics to control them)
front axle sub-frame/cross member might be different for the heavier and longer diesel engine.
I good idea would be to install the diesels fuel filter too, diesel engine is very sensitive for impurities in the fuel.
I haven't done this kind of a swap, I'm just guessing that these are the things that could very easily end up on the "to-do"-list. I did once change a different type of gearbox to a POS Ford van, and ended up changing the following things: gearbox obviously, shifter, flywheel, clutch, propshaft. Then I had to modify the gearbox support cross member and the cabin floor. All this just to change to a different type of gearbox. So my advice is: don't by an engine alone, you'll most likely need lots of other parts too. That's why a wrecked van for a part donor is a good alternative.
Merry Christmas to you too.
PS. Have you tried to ask this question from Aussies? They have loads of different older Mitsubishis down there, maybe someone there has done this swap?
http://www.mitsubishiclubaustralia.com/forums/index.php