There's nothing like a good mystery story...
As luck would have it, I have in my workshop a fully-reconditioned, stripped-down L24 block sitting on an engine stand. I also have a depth gauge! What I discovered is quite interesting...
The block has 7 head-bolt holes along each side.
The depth of all of the holes along the passenger (right) side of the block is 26.5mm
Things are much more interesting along the driver (left) side. Numbering from front to rear, the hole depths are as follows:
1. 32.0mm
2. 29.0mm
3. 25.5mm
4. 32.0mm
5. 26.5mm
6. 32.0mm
7. 28.5mm
These measurements should be considered accurate to +/- 0.5mm. Nevertheless, there's a substantial variation along the passenger-side.
Why is this? I'd have to look at the head casting (not convenient). However, I expect that height of the bosses for the head bolts have variations as well. You also have to take into account that there are three different lengths of head bolts (the parts manual calls them 'S', 'M', and 'L'). They may have different threaded lengths, too, but I don't have a set conveniently available to check this.
It seems to me that -- properly assembled -- each head bolt should end up with relatively full engagement of its threaded length. If you get their locations of the S, M, and L bolts mixed up, you may end end up with problems. I can't find any info in the L24 FSM. It only offers this cheerful note: 'Three different types of bolts are use (So be careful when in installing).'
My Haynes manual does have a bolt diagram -- although it says there are only 2 different bolt lengths - 'Short' (S) and 'Long' (L). With that noted, Haynes says:
Passenger Side: all 'S'
Driver's Side - from front to rear: L - L - S - L - S - L - L
Maybe Tom Monroe's book, Rebuilding Your Nissan/Datsun L24 Engine' offers more detailed info. Unfortunately, I've misplaced my copy.
As a first step, I suggest that you use a depth gauge (or a 'calibrated' piece of wire) to ensure that all of the hole depths in your block are about the same as my measured depths. I'm pretty sure that they will be. It seems inconceivable that just the tip of a bolt would break off at the bottom of its hole. If one of these bolts snaps off, it's going to happen at the top of the threaded part, not half-way or three-quarters of the way down, so you should be able to clearly see the top of the broken bolt sitting at or slightly beneath the top deck of the block.
BTW, all of the bolt holes are blind. That is, none are open at the bottom to some other water/oil passageway.