I'm feeling your pain, I managed to kill my fair share of L20Bs back in the day. Not fun
Now regarding your question, I don't believe that there will be any off-the-shelf parts you can use so it'll all be custom. In saying that it shouldn't be overly difficult. You'll have to pick your engine placement and then make up some engine mounts and a gearbox crossmember to suit. Not too difficult if you have a welder and know what you're doing. Wiring should be pretty straightforward. I'm not sure what you'd use as far as a radiator is concerned, you'll just have to work that out once the engine is in. Obviously exhaust, radiator hoses, accelerator cable etc will have to be made or worked out once the engine is in.
Is your 720 a 4wd? If so you might have difficulty adapting the transfer case. I don't know much about 720 transfer cases but if there is no way then you could swap to a Land Cruiser or Hilux transfer case to make things easier. There are adapters available for these. Regardless of whether it is 2wd or 4wd a custom driveshaft will be required.
Before you carry out the conversion, just make sure it is a conversion you can get engineered or mod plated. This swap has been done several times into Hiluxs and they are a similar weight so I don't imagine you'd have much trouble but just check first. Other than that keep us updated and post some photos
_________________green200b - aka Alistair
'80 200B GX - the green200b
'81 RX-7 - 12A turbo, coilovers, 4.11 Torsen LSD - daily
'74 RX-2 - 13B bridgeport, 4.44 LSD - cruiser
'82 Hilux - 350 Chev, 4.3 Detroit Locker, 33s - 4WD
Green200B has forgotten more about 200Bs than we will ever know.