67 Barracuda Big Block K frame keep it or change it ??

Of course I haven't bought an aftermarket k either.
But agree with others, original BB, A body?
Don't mess with it too much, I might modify the idler arm mount to the '68 style and stop there.
If that's not what you want, at least try to trade/sell the car to someone who'll appreciate its rarity.
If I had the budget, the Borgeson box would be next.