I agree with Dan, there are fewer truly numbers matching 340 cars out there than most realize. While lacking the numbers drive train does decrease the value a bit, it doesn't automatically degrade it to race car status in terms of value or collectability. Also, if you want to make it faster than stock you can accomplish that just fine with stock wheel wells, no need to hack on the car.

Just one more opinion, but if that car were mine I'd buy and replace stock replacement parts that it's missing (seats, uncut rear end, etc.) and build either s stock replacement 340 drive train for it, or a little stroker version if you want some more go.

*BTW, my 71 340 Duster also lacks it's original engine and trans, I'm heading my own advice on my own car.