When you climb the mast, your entire life is hanging by a single rope: the halyard. If there is any failure in the halyard, you will fall. This will result in either serious injury or death! It is wise to only climb a mast on a halyard that you think can support your weight. If you have any questions about the halyards ability to support your weight, don't use it!
The halyard on this yacht was in good condition but the splice was a bit questionable. If you attach your gantline to the shackle, always verify that the splice is in proper condition. You can't see inside the eye splice, but you can feel inside. Feeling the eye splice will let you note any discrepancies in the core and cover. W
When feeling this eye splice, the core seemed to disappear right as it entered the eye. It felt like if the eye splice was performed by cutting the core and only burying the cover. There is no way I would trust my life to such a cheap shortcut!
The rest of the halyard seemed to be in good enough condition and of sufficient strength to hold my weight, so I simply tied the halyard to the top block of my gantline. I couldn't fit the shackle through the slot on the block, so instead I tied a bowline in a bight. The end with the shackle simply hangs and dangles aside while the knot securely connects the block to the halyard.
If you don't feel safe with a piece of equipment, don't rest your life on it! Find a way to only support your life by the components that you feel are strong enough to bet your life that they will hold well. If you can not assemble a safe method to raise yourself up the mast with the reliable components, do not climb the mast!
I personally climb my own mast using the shackle because I did the splice and buried plenty of tail into the splice. On other peoples masts, I typically tie a knot onto the top block of the gantline because their splices look weak or inadequate. If you have a weak splice on your own yacht, do consider replacing the splice (or halyard entirely) with one that is strong enough to support your life.