The toploader was one of the strongest factory production boxes made,
So much so the original design ran from 1965 through to the late 70's early 80's
They where run in everything from 289ci fords to 460ci big block galaxie's to trans-am racing mustangs.
There where also special alloy cased one made for racing and to my knowledge are one of the only boxes that you can throw into any gear at any speed..
The top loader will out last any modern box to this day even with extreme punishment...
I personally have destroyed 2xm20's & a m21 with and open diff center and a 253
My top loader has seen the same motor that destroyed them and bigger with just as much punishment over 25yrs and still running strong to the point I have worn out a shifter and a selector fork