I installed the B&M shifter in my 81 Trans Am. This is supposed to be a vehicle specific shifter.... so B&M decided it was a good idea to throw in there 5 foot long universal shift cable instead of the 36 inch cable they sell separately. Of course the 5 foot cable got kinked as I was installing it because my transmission tunnel is very tight with limited space. Shifting was very hard, I ended up buying the proper cable that B&M sells in order for the shifter to shift easily. Not impressed with the cable they supplied.
The 5-foot cable is designed to loop above the transmission ensuring no sharp bends are needed. You want the cable to come straight out of the shifter, then in a very wide loop curve around over the trans and then down to the selector lever. Too tight a loop will cause the shifter to bind and the cable to eventually break. Correctly routed they will last indefinitely. There is room for the loop with a stock transmission, sounds like the 4L70E you have installed has tightened things up some. Glad you got it installed to your satisfaction, Hot Rodders find a way to make it work.