I would agree with you. The input shaft on a C4 (non-hardened) only has about 5/8" in the splines and is less than an inch in diameter. Your driveline yoke is supported by a bushing and engages a lot more of the shaft than the input on the tranny. I don't see any way that you could be in trouble.

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Paul
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retired mechanic after 35 years
specialized in Holley carbs and Ford Automatic Transmissions