BorgWarner does have short tail shafts for T-10s, but when you start down that path, purely from a cost perspective, it might be better to look at an alternative transmission with a better fit lengthwise. If you go aftermarket, you can get much better strength and an OD that will make highway cruises much more enjoyable.
Using a 9" center will require you to set your engine relatively lower in the chassis because of the lower pinion placement the 9" R&P has. If you level the engine and transmission and drop it lower the chassis, you might be able to make your existing tailshaft work. That said, you will likely need a custom
oil pan for ground clearance and
oil volume. A lower in the chassis engine placement will be at odds with your already too-low tail shaft problem. To the extent you can level the engine in the car, you can mitigate at least some of the too-low tailshaft problem, possibly a lot!
An alternative to the lowering and leveling of the power train would be to maintain the current positioning, buy the shorter tailshaft assembly, and rotate the pinion upward to get it parallel to the crankshaft if it is not already. When you use a non-IRS rear suspension, you will want to consider placing both upper and lower suspension travel limits on the rear end so you don't pull the drive shaft out of the transmission or jamb it into it.
There are any number of 429/460 cars here that have had to confront some variation of this problem. Perhaps a few of them could chime in with how they pieced everything together.
I know a transmission change is an unattractive alternative, but something to consider: the 429/460 engine family are torque monsters. That T-10 was never envisioned as living behind one of those engines. Purely from a longevity and reduced transmission service and replacement perspective, you might want to consider exploring a stronger alternative.