There should be some instructions in the shifter kit. You need to follow those exactly as it says. Sometime it takes a bit to dial in the cable but it will dial in providing you have the correct shifter for the transmission. I have a TH400 with the Quicksilver shifter.
The cable should be coming straight out of the shifter and through the floor (No Kinks) then curve around to the passenger side, around the the top the tail of the trans and around to the shift lever. You will need to be sure you are using the correct mounting bracket and shift lever bracket. I think (if memory serves correct) you start with the trans in low, making sure the little round piece slips into the shift lever bracket then move to park. Then move to each gear making adjustments back down to low. keep doing this until it slips into the shift lever hole fairly smoothly in every gear.
I've use B&M shifters ever since ever. I've been so happy with them, it's all I've ever used. I stick with what works.
I hope this helps. PM me if you need any additional help.
Edit: I just noticed. If you are using a 4L80e trans your shifter needs to accommodate the 4 speed automatic. I'm guessing you have figured that in and removed the additional pin to accommodate.