On a wristed radius arm you only have one side pivot. The other side stays the same (stock solid arm). You need at least one arm solid otherwise the axle will wrap under the truck.
I run 38.5" Swampers with 5:89 gears locked up and a built 302 with no problems.
Sure the big block will increase the torq and you might bend the one solid arm some day but you can help this by reinforcing it with some angle iron. I didn't reinforce mine and have run it for 5 years now.
Long arms do almost as well as a wristed arm in flex but will cost you more cash instead of some cheap scrap steel. There are always pros and cons to each set up.
Cage and Duff both make a nice set up that you will never have to worry about.