If I am not mistaken you can play with the stretch with the yaffee block. The hhi has 2inches out and 2inches up built into the kit so no wiggle room there. If you don't like the extra stretch this may not be appealing. The hhi kit looks very easy to install but the extra fab in the yaffee kit isn't a big deal for a guy that works with metal. I would lean towards the block because of the stretch.
We've done both and the HHI kit was hours less of work. The Yaffe kit has you make sheet metal panels to cover it up. HHI kit is easy. Cut off neck. put new neck on and weld. Make sure either way everything is competely squared up.