A CA18DET will easily fit into the 720 engine bay. You will need to replace the rear diff assy and tailshaft to more suitable items.

As for brake upgrades I dont know what struts could be dropped into the 720. So you would probally have to adapt new rotors and calipers to the 720 struts.

A guy at work has a 720 as a work car and parts are extremely difficult to obtain. They were popular in their day but hold little value now. I dont know where you are located but in WA you would be lucky to find one at the wreckers.

A CA18DET goes well in the series 3 bluebird so I expect a 720 ute to go just as well. But because it is not a project that to my knowledge anyone has undertaken you would need to Fab alot of components yourself and you would certainly need the input of a automotive engineer to ensure its all legit.