HI Eric,
I'm not sure what your mechanical abilities/budgets are (I will assume both are excellent given the magnitude of what you are undertaking), but deviating from the standards 351C/ZF is probably going to create a lot more issues and narrow the parts source and expertise options. There are a lot of 351 Clevelands out there (along with bell housings, clutches etc.) that have all been figured out and are known to work and fit well in the Pantera. Not saying that you can't do an awesome 4.6, but part of the Pantera magic (IMHO) is the rumble of the 351.
Good luck with the project!
Mark
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