Rope seals work. They did for about 75 years. There are two keys to success. 1.) minimize blow by. You do not want any vapor pressure in the crankcase. So cylinder wall prep and good ring seat is critical. 2.) As mentioned, they need a bit of compression at installation. So get it in tight. Find a PVC pipe close to the diameter of the crank journal and clamp it in place with the seal in the groove. Take a piece of decent hardwood and make a tapered square end that will fit the seal/groove. Trim seal on one end maybe 1/8" above mating surface. Use square hardwood to tap end down flush. If that went OK, do other side. Remove "fuzz" from tapping.
Vaseline is all I ever used on the seal face against the crank. Nice film of Vaseline and the crank should turn by hand, but not easily. There should be some drag with the caps snugged down. If all goes OK, you can do the rods and other mains. By the time it is all together, you will have a hard time turning it with a breaker bar.
I can't remember for sure, but I seem to recall that there is a slinger collar on the crank just ahead of the seal area ... Some old fattie I worked on years ago had a pressed-on slinger collar that was left out. It lost a fair amount of oil before we figured it out ...
Those old Chrysler flat-heads are tough. They actually run pretty good. Richard Petty won a lot of races in the early days with that little six. They can go. And I've seen one with a turbo on it that is actually quite entertaining. Won't knock a BBC down, but it runs well
There is a cooling water distribution tube that needs to be in the block to get proper cooling to the rear cylinders. The exhaust exiting through the block adds to the heat load on the cooling system, so gotta pay attention to that. Many that were run on straight water will have the cooling tube rusted away and they will all overheat ...