Installing a 360 into a ’84 5th Ave is not a big affair, but will require some parts, first.
Externally, the engines are almost identical – which means pretty much everything on your existing 318 will bolt right to the 360 – with a couple of exceptions:
The exhaust manifolds have larger ports (both on engine side and exit). I would recommend using the larger 360 manifolds and getting a new exhaust Y-pipe made that is slightly larger (maybe make the pipes ¼ inch larger (5-6 mm). That way you can get rid of the front two catalytic converters (if possible – not sure what emissions laws you have to work with there).
Engine oil pan. Just find any car (not truck) 360 (center sump) oil pan. The 318 oil pan will not come close to fitting. One like this one would work great
Chrysler Mopar 1971-80 Hot Street Rat Rod Oil Pan Raw Steel 360 Center Sump SB | eBay
Note: you may already have a usable pan. Just make sure the sump looks like this (and not to either end).
A “balanced” flexplate, so you can retain your existing torque converter (this is the best option, instead of replacing torque converter with one already balanced for 360 eng)
B&M Automotive 10236 Flexplate FLEXPLATE MOPAR 360 W/TF727 TRANS.
The right side engine mount is wider on 340 and 360 engines, by about ½ inch (13 mm) – going by memory. You can use longer bolts and washers to compensate for the difference but finding the correct right side mount would be better
73 74 75 76 DUSTER 318 340 360 ENGINE MOTOR MOUNTS "PAIR" | eBay
Your existing transmission is more than adequate for the 360 – just as long as you use a 360 balanced flexplate (or a 360 torque converter). The flexplate is the recommended option.
Your existing 7¼” differential should hold up to a 360 2-bbl or a non-high performance 360 4-bbl – but is the weak link of the entire package, as you have planned.
Getting an 8¼” (or larger) differential and at least have it under the bench, might not be a bad idea.
Changing the differential gear ratio will be a big help, especially if towing. Your existing gear ratio should be 2.2 (or a slim chance of 2.4). Finding gears in the 2.9, 3.2 or 2.7 range will help, a lot – maybe as much as the larger engine will make, if not more.
Your engine pulleys, P/S pump, A/C compressor, alternator, A.I.R. pump (if to be re-used) will all fit just fine.
It appears you will need to find a 318 or 360 distributer (with vacuum advance) and ICM (Ignition Control Module). This (or one like it) should fix the ignition problem
Proform Electronic Ignition Distributor Kit Mopar Dodge Chrysler 273 318 340 360 | eBay
A 4-bbl will let the engine run better – but will also come with additional parts to acquire, as well (Intake, carburetor, transmission kickdown linkage, air cleaner, and so forth).
Note: the 360 2-bbl carburetor and the 318 2-bbl carburetor are completely different, as well as the bolt pattern on the intake manifold.
If your existing 318 was Lean Burn - but no longer have the computer for it, then finding a non-computer controlled carburetor will also be on your list of things to get. There is a chance your 360 might also be computer controlled (will need to see a picture of it to tell).
Before installing the engine – I would recommend changing the freeze/expansion plugs, replace most of the engine seals/gaskets including valve seals (head gaskets don't need changed, unless heads are removed) and at least check the timing chain for wear before installing. Once done, the engine should be good to go for a long time.
BudW
Note: the links I have above are from eBay. I just found them to use as an examples. Using that data, you may be able to find a source local or more cost effective to get to you. I am not promoting those links.