Will 70 Dart V8 K-member fit 65 Barracud?

no, won't fit. too wide.
a '65 K frame isn't motor specific. /6 and V8 the same.
All you need is 273 motor mounts
That’s good news. I just assumed I’d need a stronger Kmember for the V8. So it’s just a bolt in if I have the 273 mounts?