Brian,
as far as I know, thats a National only seal. I dont think there will be a C/R or other seal that crosses to a different number. Timken uses the same part number, 450621. Napa should be able to cross to a National seal, or just go to a fluid/hydraulic repair center in your area.
Here are the dimension for the Milodon #13563 seal, which is a National 450621:
450621 Nitrile, Shaft diameter- 2.156,3.196 OD 0.437 width
You should be able to find one.
Todd
RIP Monte Smith
Your work is a reflection of yourself, autograph it with quality.
WD for Diamond Pistons,Sidewinder cylinder heads, Wiseco, K1 rods and cranks,BAM lifters, Morel lifters, Molnar Technologies, Harland Sharp, Pro Gear, Cometic, King Engine Bearings and many others.
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Brian, your if your bore diameter in the front cover measures 3.188" the stock seal will work.