I had my '53 Chevy crush a pair of cinder blocks back then as I was just getting the car situated on them. The holes were horizontal, and there's not much strength that way. Concrete blocks are much stronger, and I always position the holes vertically when I use them (which is not very often any more).
One other bad experience I had was while using jack stands with 3 legs and they tipped over with the car on them. The wheels were off and one stand leg went through the trunk floor and punctured (big time) the spare tire, the other jack stand leg just went through the trunk floor. You guessed it - they were aimed the same direction. And if you didn't guess it, always have a leg on one jack stand pointing toward the front of the car and one leg on the other stand pointing toward the rear. Why? Three legs = 120 degrees spacing. The leg pointing toward the front is 90 degrees from the center of the jack; the other two legs are only 30 from the center of the jack. It's not terribly hard when reefing on a rusted bolt to disrupt the balance of the stands, and you either move very fast or become another statistic. Thank God, my friend who was under the car saw it start to move and scrambled out just in time! I still shudder when I think about it, and that happened almost 50 years ago.
<font color=green>Duane</font color=green>