If using the supplied block is in the rules and you want to respect that rule you can simply use a veneer cover if you want a different wood grain. Leave ~1/4" of the original block for the frame.
I look for quarter sawed blocks. I like to go to hobby lobby or Michaels to inspect the revell blocks because they are in see through plastic. I look for the grain of the wood to be horizontal. Quarter sawn wood is more stable than rift sawn or plain sawn. I also bake my blocks at 225 for 4 hours then shut the oven off and leave them in there until cool.
Its my kiln process.