you need to look at the math and you'll see the answer.
mc5,6,10,49 all have a pin to crown of 1.00.
mc20 has a pin to crown of 1.06.
so a mc20 can run a mc5,6,10,49 piston because the earlier style piston has a shorter pin to crown ht.
the top ring land will be nowhere near the top of the bore.
this is because the mc20 block is taller than the mc10 block it evolved from.
the piston might be a little low in the bore (.06) so the compression will suffer a tiny bit.
but that's not a deal breaker to make the engine run and the port timing possibly might be a little better.
reversed mc20 piston in mc5,6,10 block will not work. the ring land would clear the bore top and end up hanging the rings.
mc20 piston in a mc49 block is possibly doable because i'm pretty sure there's more crush space in the mc49 block.
easiest way to see is assemble piston,rod assembly , assemble short block without torquing rod bolts but tight , turn the piston 360 degrees through full stroke and return. if the top ring doesn't come clear you are fine. if the top bit of piston comes clear you'll need to add the correct gauge head gasket to compensate.
hope this helps
d
Dave L.