340 to 273 tranny Question

If it's a steel crank 340, you should be able to use your original flexplate. If it's a cast crank 340, you will need a B&M plate for the cast crank or have weights put on the converter.

I don't recall what year they changed, but at least the earlier 273's had a smaller snout on the converter. I found out the hard way when I used a rebalanced 273 steel crank in a 340 and my converter snout wouldn't fit into the back of the crank! So in your case, make sure you don't have the smaller snout converter.