[Fr. Frank]

NEVER, NEVER, NEVER use concrete blocks stacked on their faces, or flat surfaces. They' break more often than not.

To support my transom, I stack 8"x16" 16lb blocks three high on each side, stacked horizontally, with flat sides perpendicular to the ground.
I place them under the corners of the transom, with w/ an 18' long section of 2"x10" on top, and a 16" long 4"x4" on top of that, placed at the outer chine, so that 2" to 3" inches extends beyond the rear of the transom, and 1/2" extends beyond the hard-chine along the length.
This allows me to tilt the motor all the way down without hitting the ground.

I NEVER crawl or work under a boat I have not tried to push off the blocks by hand. No Kidding!! I will put both hands against a hull and push and pull violently to make sure it is stable and secure, and even then I am reluctant.

After blocking a boat off of a trailer, I push hull supports in from the sides that look like this: They're made out of 2"x8" boards. This prevents the boat from rocking, and makes it safe to climb aboard and work.