I'm not 100% certain (pistons are 4500 miles away) but I think Kawasaki 900 or 1000 pistons will fit 250 rods (I haven't looked at parts since late 1970's~early 80's)
250 block will need re-linering to go more than 3mm oversize ( max 59mm before liner gets too thin)
360 block will drop onto 250 bottom end but I'm pretty sure small end is smaller on 250 (hell, I haven't modified a 250 since the mid 80's)
You may be able to find pistons from a modern 4 valve 600 or 750 that will fit 250 rods. They will need less machining than early 2 valve pistons and you shouldn't have to fly-cut combustion chamber with a 'flat top' piston.
Personally, if I was doing 250 mods today, I would get a GS850 block, shrink out liners then fit 850 (69mm piston) liners into 250 block. Bore for 1mm OS GS850 pistons and you have a 391cc motor.
It could do with 1mm oversize intake valves but works fine with stock parts (120mph)
The crankcase will need machining, I just bolted mine to lathe saddle with spacers and used boring bar in chuck to open 0.020" larger than liner.
It's a non-critical dimension so set-up didn't take too long
I'm probably the first person to do a real big bore conversion on 250 motor, most people started with 360's. If you want to go 'huge' you need to fit GS1100 connecting rods and modified 72~74mm pistons