If it weren't such a pain to change the seal once the car is up and running, I would say go for it. Keep in mind that by doing so you are only bringing the top and bottom of the circumference closer together. The sides remain at nearly the same distance apart.
Why not replace the seal holder with an after market unit. Hughes sells a nice one that uses the Superformance seal.
1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)
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