Sort of moving off subject a little. I'd have to check but I think the cab will bolt on pretty much straight up. Not sure about the tub / tray. Other things to consider is which engine / gearbox combo you plan to run. Bolting the J15 engine and box into the 720 chassis may have issues with mounts and tailshaft, not to mention the column shifter
.If using an L series, or anything else, you may need to cut the rad support where the radiator bolts up and use a larger radiator pushed further forward due to the extra length. If you compare the two cabs you'll see what I mean. American 620's had a different rad support panel then ours, all ran L series engines.
620 diff from memory is a little narrower than the 720, so if you want the 620 ratio and alloy housing, you'll need to swap the centres over, unless you want the hot rod look with deeper dish on the back and run the 620 unit complete in the 720 chassis.
I've been thinking about doing a similar thing with the project 620 I have, sitting it on a 720 4WD chassis as my daily drive. Diesel or petrol is the D I have to make