Paul.  What follows is theoretical.  We need someone like Ted, Tim McMaster, or John Mummert to indicate IF there is merit to the thinking.

IF the crank in your block was turned undersize for reconditioning purposes by a machine shop, might the crank set-up have been SLIGHTLY 'off '?  IF not chucked up correctly, could the real seal surface NOT be EXACTLY concentric with the remachined main bearing surface?

Again, I'm suggesting one cylindrical surface being slightly out of alignment with another by mere thousands of an inch creating a minor cam effect on the sealing surface.  In initially assembling and running the engine after assembly, the rear seal would tolerate the discrepancy for a period of miles.  Seemingly, a concentricity discrepancy would take longer to wear a pliable neoprene seal than it would a fixed rope type (packing style) seal.
Doubt all of that happens, most people just arnt as good at fitting new seal as they think unfortunatley, i have 4 y block cars, all rebuilt at one time, one with turned down 312 crank also and nary a drop of oil from the lot, 2x4, tripower, 4/71 blower and single 4 bbl.

IF you proceed to replace the rear seal yet again, the concentricity of the rear seal area could be verified using a dial indicator mounted with a magnetic base to the oil pan rail.  The checking would be done after the aluminum seal retainer was removed and before doing anything to the main bearing caps that secure the crank.

Again, this is an unusual possibility suggested by the number of rear seals you've had fail over different mileage periods and by my machining experience which does not include the operation of a crankshaft grinder, but a lot of engine lathe set-up and machining work.

Paul.
Thanks for sharing in detail the history of your rear main seal woes.  It would appear that you’ve tried the majority of fixes.  At this point and with a fresh load of leak detecting dye in the oil and with the car on a lift with someone watching under it, start the car and look for the first place the colored oil comes from.  Your oil pressure attributes mirror those on my '55 Customline and with 145K+ miles on it now, rear main seal leakage is minimal if any at all.  With that in mind, I think you can rule out the oil pressure being the culprit.
  
Were you using the black neoprene seal before going back to the rope type of seal?  We likely already discussed this but have to ask anyhow.  I’ve had the older orange seals wear out very quickly but have had very good luck with the newer and harder black Best Gasket seals that replaced the orange seals for the 312 engines.
 
While it’s been brought up about the concentricity of the seal surface versus the rear main journal, it would have to be off a bunch to be a sealing problem.  Whenever a crankshaft is being turned, it is possible for the seal surface to be slightly out of alignment with the journal and if the crankshaft has been turned multiple times, then this alignment (concentricity) can be off even more but unlikely to be off enough to be a rear main sealing issue.  Haven’t seen that yet but there’s a first time for everything.  Whatever amount the seal surface is off will be the same as the rear flywheel flange so any misalignment can be checked for at the rear of the crankshaft also.  I have cleaned up the 312 seal surfaces as much as 0.010” without any issues in regards to the neoprene seals being a problem so there is a given amount of latitude in that area.
 
If and when the engine is pulled down, look at the rod bearing wear.  If the bearings are not wearing squarely across their surface and the wear looks to be crooked or lopsided between the top half and the bottom half of the bearings, then the crankshaft may not have been ground in a grinder that was 'not' parallel between the ends.  In this case, you’re dealing with a rear seal surface that’s not parallel with the main journals.
 
It boggles the mind how the first generation of Flathead V8s did not even have a rear seal and wouldn’t have a leakage problem until the blowby was bad enough to actually push some oil out.  Now with all modern engines having some kind of a positive type of rear main seal, there’s a major oil leak if all is not perfect.