The Ronstan stuff does fail in an internal boom, we have used the friedrecksen wire blocks and they blow up after a few days sailing, they are rated high enough (cant remember exactly) but they just dont last. The Harken carbo blocks blow up as well even when used on the big rope section.

The Harken wire blocks are very well made and have not failed us yet, most guys use them for internal booms.

The Marstrom (playskool) blocks are good when you first get them but they require a lot of maintenance and they are also prone to breaking the side of the block surface off.... we had this at the europeans and it really screwed our day. I am now running the 51mm Harken wire sheaves and they have less friction than the Marstrom blocks and are indestructable.

attached is a pic of the rear of my boom, more pics to follow.