This doesn't solve your problem, because I wouldn't do it with the original engine. Mopar Muscle had a good article about stroked small blocks. You can get 400+ cubic inches with a 360 combo.
I too have a 340 Challenger, and have always dreamed of extra HP. Naturally the RB engines is the 1st thought.
After reading the article, I'm definitely going the "stroked" small block route to get all the HP you need.
Unfortunately this will be in the way distance future for me.
Good luck