I noticed your setup w/garden hose on your video...this morning at 5-ish
two other flathead videos had no visible cooling

I'm pretty sure I didn't do (much) harm
read a few internet stories on dry block running
like:
"...we run a sbc on pump gas no water no block fill we just keep our eyes on the engine by feeling the front and back of the heads with our hands if it burns its to hot! BUT we only do two meets a year and we make maybe four runs a day with cooling down time aplenty we have been running this engine 350 in a t altered for atleast 10 years no problem sometimes we run nitros sometimes not 11s without the gas 9.6s with NEVER damaged it..."

my fingers are crossed

 

If you only ran it for a very short time then i doubt it. Probably had water in it (hopefully). I doubt very much it got hot enough to crack in 30 seconds but i never run them on the ground/tire/test stand without at least filling the block with water first and only for a short time to verify turning over/starting/popping/backfire/etc. I'm not sure where to get the garden hose adapters but i'll see if can find out. They are very inexpensive (like dollars, not 10's). They leak a little but they work for their purpose.

 

If you ran it for thirty seconds only, I seriously doubt if you damaged anything. I would suggest you not do it again.

 

I just checked the video
it ran for a full minute

so stupid...

 

There is this phenomena called Thermal Equilibrium. Things will want to seize without coolant.