Actually you can use the early rear steers on these trucks with zero ill affects. It has nothing to do with safety why people chose the front steer unit. Rear steer clip can affect your header choice which could also affect the choice to run a big block. It has nothing to do with safety it has to do with tight spaces.lol All clips can be unsafe if install incorrectly.
Here is the what I did on on AD truck to resolve the the ujoints. I install a double d on the box and ran a ujoint. With a little engineering you can use almost anything safely. None of this stuff is plug and play you have to be creative sometimes.
That said, I do believe the 70-81 Camaro/Firebird is a better choice for the 55-59 Chevy truck but it has nothing to do with safety.