What does this mean? Too tight piston clearance?
that has nothing to do with your blowup.
i wouldnt worry about it.
That's exactly the place oil is supposed to burn.
Why like that only on one side?
R5/RD 350 head squish properties are horrible and usually never the same side to side.
For any performance builds... 1st thing that gets done after a fresh crank, seals, and a bore with new pistons is to the machine heads and deck to get the heads up to modern spec's to run on this ethanol based unleaded fuels we have today.
If you just mill the heads to tighten up the squish, then the compression ratio is way too high for pump fuel unless you turn it into a dog by reducing the ignition timing.
Plus, you just went through a blowup and as one cylinder starts to have a problem with seizure the other cylinder see's a big gain in load and creates extra heat in the chamber and will show a difference your seeing.
I do stagger jet a lot of my RD builds with different main jets left to right.
Chuck