Chip,
I measured my 235 block and I found the top surface of the oil pump to be about 3-1/64" below the machined lip in the block. (It was kind of hard to see down in there because my block is parked under a workbench)
The 1/64th came from where the gear wore into the top of the pump. It looks like new clearance should be 3" exactly. Note that this is a '57 block with the large journal cam, so I don't know if the '54 with its small journal cam was any differant. I'm pretty sure this is a stock cam in this engine, and it is a truck block with solid lifters.
There is another 1/16" to the outer machined surface. The shoulder machined into the block is what the mounting bracket/clamp rides in. The top of the pump and the gear seem to have been in constant contact over the 20k miles+ that are on this engine, a '57 block that may have been rebuilt at least once in its lifetime. The distributor itself shows very little wear although you can tell from the gear which way it rotates- there is tooth wear on the driven side of the gear and shiny spots on the driving side of the oil pump tang. The gear is installed so the long shoulder is on the oil pump side, although I expect that if you installed it upside down it wouldn't go in all the way anyway.
The distributor body itself has absolutely no shoulder to positively position it in the block. The clamp provides positive retention against twisting but little force in the vertical direction. The dist. simply sits in the block. It looks to me from everything I can see that the oil pump is supposed to provide the bearing surface for the distrubutor, and that the failure you experienced was either from lack of lubrication of that surface or from the thrust bearing in your HEI distributor providing too much downforce on the gear/top of pump. I think that if you re-machine the HEI so it can use the original style clamp and replace the gear and oil pump you will be fine. I also noticed that there is a flat machined into the dist with an oil passage to allow oil in/out of the bottom bearing in the distributor. I can take some detailed pictures of the distributor if you like. I can also try to take a shot down into the distributor hole.
The '50 shop manual states (for the babbit pounder) that "The distributor housing is designed to pilot down into the right side of the cylinder block which supports the distributor and provides a bearing for the distributor shaft." That seems to indicate that there is supposed to be thrust on the oil pump but yours must be too much.
