Unless you saturate the seal with oil and roll it out to a considerably thinner cross-section, you will never succeed in pulling it through that groove. If you're not willing to try that approach, use the rubber seal. The small gaps behind the seal won't cause a problem as long as the lip fits the crankshaft snugly. Some people like to rotate the end of the rubber seal slightly out of alignment with the block/cap parting line, maybe 1/8" or so to minimize the possibility of getting a leak at the block/cap intersection.
Jerry
