I don't know. There were many performance engines in A-Bodies from Hyper pack sixes to 273s, 340s, 360s, 383s, 440s and Hemis. How many more do you need? When the 318 was a performance engine the real big guns were gone. 340, 440, etc.
Also, Chrysler was aware of cutting their own throat. Would a 4bbl 318, which would cost less, cut into 340 sales? Without a doubt it would. So why spend more to produce another engine that will lower revenue?
I think they hit the sweet spot with the 340 Darts Dusters and Demons, and they sold a lot of them. I'm pretty sure whatever form of cost/benefit analysis they did said to leave the 318 as is.
