how do i make my /6 sound good...

i have received a lot of good info for this new slant 6 motor that i am dropping in my 71 dart. i have a road block now. i have a '64 /6 engine and a '71 904 trans, therefore they don't mate because the torque converter is too big for the crankshaft inlet. Should I A: get a custom torque converter, or B: get an older 904 trans? what's the best way to resolve this?