7 1/4 Why is it so weak?

The Cheif nailed it and good. While it is possible to strengthen the parts through cryogenics, the cost factor vs strength gain is not worth the effort. The actual part size is very small for the power given to it. Small V8 power will usally destroy it in a short time.

The 1 inch larger rear (8-1/4) is fine for 400+ HP behind a mild big block like a 383 or 400. A 440 will be OK if you leave the power plant alone. It makes enuff torque to kill it if you hammer it alot.
I've seen 500 Indy headed big blocks dish out some really low 9 second runs with an 8-3/4. Only a 1/2 bigger then the 8-1/4. Capable of much more power.

Both of the 8+ sized rears are easy to get with the 8-1/4 being preety cheap. A new sure grip unit is under$500 IIRC for the 8-1/4. Easy up grade.