converter help fast answer needed

I agree - lock-up converters must go with lock-up trans. Not sure why it won't work, but it won't. Lock-up transmissions have a machined end on the input shaft. Non-lock-up are splined all the way to the tip.

Also, if the 340 has a cast crank/externally balanced, you'll need the correct B&M flexplate to use with a neutral converter in a compatible trans.