Here is a picture of my engine upside down on my eng stand. The rear main cap is still on. The seal is under main and embedded in block under the crank. If I had a bad real seal and the engine had not been rebuilt for many years I would have pulled the engine for a rebuild.

All the mains and crank have to come out to replace the rear crank seal. That means disconnecting all the piston rod caps. Mixup the caps and you get piston rod slap (out of round rods).
BEEN THERE, DONE IT.

I would do a stock rebuild of the existing engine.