Picking Up 340 Short Block - What to Look For?

I agree with RRR,

Is the block all your getting?
Are the main caps original or changed.
Bore sizes proper for what they are selling you.
No cracks. Chase the threads by the timing chain cover. Or chase all the threads.
Bring a straight edge and feeler gauges and check head mounting surfaces.

Honestly there are a lot you can look for before you buy but it is a 50 year old part.
Risk vs reward.
Good luck and enjoy the road trip!