For what it's worth, I'm running a 351W, C4 combo also. I've got a narrowed 9" in the ass end tho.
I purchased a FMS aluminum driveshaft last year, and thought (after removing the rear flange and having the proper U-joint installed, to fit the 9" flange), installed it, and thought it'd be too short as well.
Well, it goes in enough, apparently.
While personally I'd like it to go in further, the couple of inches that it does go in, must be enough.
I would highly recommend replacing the transmission slip yoke bushing tho.
(just to cya).
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