It looks like a standard 3/8" internal boom bail. Probably no worries about the bail itself failing; the concern would be about the bail pulling out due to metal fatigue of the aluminum boom. I have not installed one of these bails, it seems to me that you would cut/drill the holes to fit the part and then slide the bail down from the end of the boom. Run a fish wire down and wrap it around the bail before sending it down through the boom so you can pull the eye out when it is in position. The bail should have threaded fittings so you don't have to hold nuts on from behind. www.ballengerspars.com
shows both 3/8" and 1/2" internal bails on their parts list. Give them a call and ask about suitable sizes and if it would be a good decision to add a second block/bail to your boom.
As for the size of the block hardware; when in doubt it's always best to oversize. If you do double blocks on the boom you could go with the 57mm blocks on the boom because the load is distributed to two separate load points. So for a 6:1 you could do a 2602 double, 2600 single, and the 1564 triple.