Pulled the reed valve on my new to me '74 360DT, will check it carefully for cracks/gaps. OK to put in ultrasonic cleaner (I would imagine no heat best to avoid warping little doors?) Thanks!

All I would be concerned about is damage to the rubber coating on the block.  There's not much need for an ultrasonic cleaning really.  Unless it was left open and is full of dirt and leaves it should be pretty much spotless.

You can remove the reed petals and wipe them clean, make sure there is no grit or debris in the block.  That would be plenty thorough.