While picking up 20 mph is significant, it does sound like something is still not right. Final drive on the C-4 is 1-1 as was mentioned, and with your gearing, the car should make a lot more mph than that before it runs out. Does the converter feel tighter? I would think it would be a pretty noticeable difference considering what you started out with. I got lucky with the converter in the Buick, my mustang took 3 converters and 2 C-4's (one built local, then a PA because I was fed up) before I had a tranny/converter combo I was happy with. Once it's right, you'll wish you dropped the 5 speed sooner, but until then, it's going to piss you off.