I don't think the rope and the rubber seal are interchangeable. You get both seals in the gasket set so you can use whichever one fits the rear main cap and the block.

It's not uncommon to have a little drag on the crank with a newly-installed rope seal. It "wears-in" very quickly, forming itself into a leakproof fit to the crankshaft. It wouldn't surprise me if you get a big leak from using a rubber seal where a rope should be.
Jerry

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