Why would you spend a bunch of money to make a 4k block take 5k pistons that wont work with the head that comes with the 4k anyway, and you've spent 3 times more than a 5k anyway...... Plus sacrificing block integrity by machining the shit out of it....
In an effort to actually answer this gentleman's question:
Timing chains and gear - Yes it will fit. 4k ones are better than 5k
Oil return - Think i can use 4k - Yes you can
Sump - think i can use 4k? - Yes. 3K ones wont fit.
Gasket kit - $60 ebay - Head gasket might not work with the bore diameter. Buy a 5K one.
Valves / Springs / Rockers - Have complete 4k head and 3k head only - Nine times out of ten your 5k is a dished piston. Which means you need the correct head and valves and springs. Best valve springs for a 5K performance setup are probably crow cams 4220s. Forget the bullshit about kingswood springs and glockenspiel retainers etc.
Push rods - use 4k? You should get a 5k head with 5k rods..
Timing cover - use 4k? Common to all
Rocker cover - i can use 4k - Common to all. Early k/2k/3k ones are the nicest looking with less mounts on the top.
Pulleys - Depends on your water pump. There are 3, possibly 4 different water pump pulleys, so you need to make sure everything matches. This is the last thing you should worry about.
Thermo and thermo housing - Interchangeable. Depends what inlets/outlets you need for the car, swap your 4k one over for EASY
oil filter mount (not sure correct name of this) - use 4k? - Same
Engine mounts - have new 4k - Same
Intake - lynx manifold and twin su's already have - Same
Cam - regrind a 4k one or are they different? - Depends on whether you are running hydraulic lifters, they have different grinds. Dumbasses will tell you that you need to go solid, so you will spend your weekends trying to match Mercer Raceabout lifters to Studebacker rocket covers. Don't.
Alternator- Interchangeable, but depending on the belts and pulleys (see water pump comment) you need to make sure your alternator bracket (top one) and the belt length is correct. Not hard though.
Distributor - Interchangeable.
Starter motor - Interchangeable, biggest issue is the headers.
Flywheel - 5K ones are different. I can't remember why.
Water pump - See above comments. All are interchangeable, best to just swap pump/pulley/crank pulley/alternator/alternator brackets off your 4K for EASYFUNS
in summary, most stuff will swap over but the head and valvetrain will cause you more grief than is necessary.
5k's are not rare. Both of mine cost me 100 each. full motors in pieces. hold out for a complete one and swap the fruit, or chase down a head to suit your short block